First of all, I know you're referring to the first Zionist Congress, but the Zionist movement itself had begun before. The first people to write about Zionism that books usually mention as influential are the German-Jewish Moses Hess (1862, wrote Rome and Jerusalem arguing for a Jewish social commonwealth in Palestine to solve the "Jewish problem") and the Russian-Jew Leo Pinsker (1881, wrote Auto-Emancipation, said anti-Semitism was everywhere and Jews had to organize and find any national home). In any case, the first real Zionist organization most books talk about is Hovevei Zion (Lovers of Zion) which was formed in the 1870s, and would turn into Hibbat Zion (The Love of Zion) in 1887 and adopt Pinsker's writing as their "ideological charter".

Both Hovevei Zion and Hibbat Zion were small, and when reorganized into Hibbat Zion Pinsker was essentially forced to be leader by the group. It received very little funding, including from world Jewry, and came out of Eastern Europe/Russia. It failed to attract any major attention, or encourage much immigration: only a very small fraction of the Eastern European Jewish population turned to Zionism.

Another group working around the same time as Hibbat Zion was the BILU (acronym refers to a biblical phrase). They organized and committed to establishing agricultural settlements in Palestine (they were based in Ukraine). They too had very little success in organizing, and sent 14 members to try and settle in Palestine. They failed, and were forced to return to Europe.

At around the same time, a decentralized movement to a degree arose. Many Jews were fleeing for Palestine from Russia and Eastern Europe from 1881-on, in what came to be called the first Aliyah. Some fraction were indeed Zionists, but most were simply fleeing anti-Semitism. Those who were Zionists managed to appeal to wealthy Western Jews, like Baron Edmond de Rothschild and Moses Montefiore, who helped keep afloat some settlements. During this first Aliyah, the first wave of immigration (1882-1903), roughly 20-30,000 people headed for Palestine. Most would leave for the West, or return to Russia, but the movement did help teach organizations what not to do when attempting to settle in Palestine. Hibbat Zion's various societies (it was a confederation) were in decline when Herzl finally organized the Zionist Organization at the end of the decade.

Now that we somewhat understand the background of the Zionist movement, and how it played into the Zionist Congress in 1897, let's talk about the rest of the world. How did they react?

I can't find any initial reactions to the Congress itself, but I can tell you what followed in the reactions to the organization of the Zionist movement. The Germans were fairly sympathetic from the start to the movement. Kaiser Wilhelm II wrote in 1898 that perhaps "the tribe of Shem would be directed [once embarked on the Zionist road] to worthier goals than the exploitation of Christians." True, the Jews had "killed our Savior." But, given "the tremendous power represented by international Jewish capital in all its dangerousness," it would be well were the Jews to look upon Germany as their pro-Zionist protector.

Wilhelm II wasn't the only one paying attention of course, but he did offer great power recognition that the Zionists desperately needed to lend legitimacy to their cause. Herzl's plan was to meet with the Ottomans, attempt to convince them to allow for a Jewish commonwealth in Palestine, and if he failed, to attempt to get a great power to do the convincing for him. He met with the king of Italy, Pope Pius X, Kaiser Wilhelm (twice), and the Ottoman sultan. The Kaiser had indeed attempted to give full support to Herzl, and even wrote that he would intercede with the Sultan. However, when meeting with the Sultan, it's noted that he tried to raise the issue of Palestine, but could get nowhere. Count Eulenberg noted to Herzl that "we are anxious to remain on good terms with [the Sultan]. As a guest, the Kaiser could not, of course, press the subject."

At the same time that the German overtures were being made, the Ottomans were also being approached, of course. The Sultan passed along the message that the Ottomans would never part with any province's sovereignty, especially not Jerusalem. Herzl tried to present another offer to the Sultan, saying that Jews could perhaps establish an autonomous vassal state, under the suzerainty of the Sultan, where the Jews would embrace Ottoman nationality, pay a tribute of 100,000 pounds (which would rise to 1 million annually as immigration increased), and they'd be granted autonomy as a result and allowed to maintain an army. The Turks turned this down as well, saying that the Jews were welcome to live in the Ottoman Empire, but they could not be granted any territory or autonomy.

In London, the British had been approached as well. While Zionism had electrified some Jews, rich Jews remained mostly aloof and unwilling to fund the Zionist enterprise (with the notable exception of Montagu). However, the British all but ignored the Zionist movement until negotiations began in earnest in 1902, and in 1903 they offered a piece of land (the Uganda Plan, wrongly named as the land was actually in Kenya). Russian Jews would reject the plan, as did some of the other Jews in the Zionist organization, and Herzl would eventually join that group. The British negotiations were notable only because they, in essence, formed a base for the future negotiations with the British on a Zionist state.

At the same time as negotiations were going on with the British, the Russians were also talking to the Zionists. It would be the Russians who first recognized the Zionist cause and Jewish right to a state, in 1903. A letter from the Russian Minister of the Interior, Vyachelslav Plehve, came to Herzl on 12 August 1903. It promised that Russia would intervene with the Sultan on behalf of the Zionists, and would assist them in the organization of a massive Jewish immigration and settlement run in Palestine with the aim of establishing a Jewish state.

This was momentous because it lent great power legitimacy to the Zionist cause, as noted, since it meant that a great power would take action with the goal of establishing a Jewish state. This letter helped sway Herzl against accepting the Uganda offer, because it gave a greater sense of legitimacy. However, despite all the support, the Zionist cause felt it needed a "concert of European powers" to convince the Ottomans, and this they failed to get. As noted, the British gave an offer, but the offer was effectively rescinded when the next colonial secretary took the place of Chamberlain (who had resigned). The Foreign Office also changed its tune, because of the strong objections by white settler Kenyans to the idea of Jewish immigration and settlement. Without the British, and with the Kaiser refusing to press too hard on the Ottomans, two major world powers were already out of the picture for convincing the Ottomans to give up Palestine, which was the ultimate goal of the majority of the Zionists. Herzl continued, therefore, his diplomacy.

He met with Victor Emmanuel III of Italy in early 1904, who received him kindly. However, he was of the opinion that the Zionist movement was attempting to act too fast, and that he had little to gain. He believed that Italy had no "Jewish problem", though he did say that "Palestine will and must get into your hands, it is only a question of time. Wait until you have half a million Jews there!". This would be especially interesting, since it was when over half a million Jews were there that the Civil War erupted in Palestine. In fact, in the partition plan in 1947, it was estimated there would be roughly 498,000 Jews (half a million!) in the Jewish state, when one didn't include the 100,000 in what would've been internationally administered Jerusalem. At any rate, the Italians would lend their support, but did not seem eager to lead the cause. The same would be true of the Austro-Hungarians (Herzl's home), who did not lend their support until the autumn of 1903. Using Russia's letter helped immensely with convincing other powers that they would not be alone in supporting Zionism, since it meant they had less fear of upsetting a great power. Herzl's death in 1904, however, ruined what most agree would've been the next step: a visit to London to attempt to establish a concert of powers to finally force the question. One of the most effective Zionist statesmen was lost with Herzl, and the movement's international support failed to gain steam again from then on, until Weizmann began to take a leading role in international contacts with Britain in 1917.

Sources:

Friedman, Isaiah. Theodor Herzl: Political Activity and Achievements Israel Studies, Vol. 9, No. 3 (Fall, 2004), pp. 46-79

Harms, Gregory, and Todd M. Ferry. The Palestine-Israel Conflict: A Basic Introduction. London: Pluto, 2005. Print.

Morris, Benny. Righteous Victims: A History of the Zionist-Arab Conflict, 1881-2001. New York: Knopf, 2001. Print.

Reich, Bernard. A Brief History of Israel. New York, NY: Facts On File/Checkmark, 2008. Print.

Smith, Charles D. Palestine and the Arab-Israeli Conflict: A History with Documents. Basingstoke: Palgrave Macmillan, 2010. Print.