First, just so you're aware, it's the Molotov-Ribbentrop pact, named for Soviet Minister of Foreign Affairs Vyacheslav Molotov.

The answer to your question is yes: The Soviet invasion and occupation of eastern Poland from 1939-1941 was brutal.

The Soviet Union called its invasion of Poland an occupation, and justified it as a necessary response to a power vacuum. They claimed that the Polish government had collapsed under German invasion, and, according to Timothy Snyder in Bloodlands: Europe Between Hitler and Stalin, "[s]ince Poland could no longer protect its own citizens, the Red Army had to enter the country on a peacekeeping mission." This was a transparently self-serving justification for the Soviet Union to grab a portion of a defeated country that it had loathed, feared, and fought for years (see, e.g., the Polish-Soviet War of 1919-20) in fulfillment of the "secret additional protocol" of the Molotov-Ribbentrop Pact.

Snyder describes the beginning of the Soviet occupation in gruesome detail:

The removal of [Polish army officers who were imprisoned in the Soviet Union by Soviet forces] was a kind of decapitation of Polish society. The Soviets took more than one hundred thousand prisoners of war, but released the men and kept only the officers. More than two thirds of these officers came from the reserves. Like [Polish artist Jozef] Czapski and his botanist companion, these reserve officers were educated professionals and intellectuals, not military men. Thousands of doctors, lawyers, scientists, professors, and politicians were thus removed from Poland.

Meanwhile, Soviet occupying forces in eastern Poland placed the lower orders of society in the vacated heights. Prisons were emptied, and political prisoners, usually communists, were put in charge of local government. Soviet agitators urged peasants to take revenge on landlords. Though most people resisted the call to criminality, chaos reigned as thousands did not. Mass murders with axes were suddenly frequent. One man was tied to a stake, then had some of his skin peeled off and his wound salted before being forced to watch the execution of his family. Usually the Red Army behaved well, though sometimes soldiers joined in the violence, as when a pair killed a local official and then took his gold teeth.

In the background, the NKVD entered the country, in force. In the twenty-one months to come it made more arrests in occupied eastern Poland than in the entire Soviet Union, seizing some 109,400 Polish citizens. The typical sentence was eight years in the Gulag; about 8,513 people were sentenced to death.

The Soviet Union moved quickly to annex eastern Poland and apply Soviet social and economic policies in their zone. Richard Evans describes this process in The Third Reich at War: "In pursuit of a social revolution, the Soviet administration expropriated Polish property, nationalized banks and divided up the big estates among peasant smallholders." Evans also describes some aspects of the Soviet occupation that seem humane even by western liberal lights: "Formal civil rights were extended to everybody, and younger Jews in particular welcomed their liberation from the antisemitic discrimination practised by the regime of the Polish colonels." As in other times and places, however, the primary levers of Soviet policy-making were forced expropriation, imprisonment, deportation, and shooting. Poles were drafted into the Red Army, and hundreds of thousands - the educated, the wealthy, the politically unreliable, all of whom "pose[d] a danger to the new order" - were deported into the USSR to live and work (and, in the end, die) as deportees in the vast Soviet hinterland or as prisoners in the Gulag. This practice was, by Soviet standards, an established policy with a long pedigree: Stalin's government had done it to the kulaks and to various disfavored Soviet nationalities at several points in the 1930s.

And then, of course, there was Katyn. The Katyn massacre was, by the warped, blood-soaked measuring stick of Eastern Europe during the Second World War, a crime of fairly ordinary proportions. Some 15,000 Polish officers held in Soviet captivity were shot in April-May 1940, and about 4,000 of them buried in mass graves in the Katyn Forest, near Smolensk. Stalin and his secret police chief Lavrentiy Beria had decided that they were too much of a political liability ever to be released. Ironically, Katyn looms large in our understanding of the Soviet occupation of Poland primarily because of the success with which the Nazis cynically publicized their discovery of the burial site in 1943. No such publicity was given by Goebbels to the many mass shootings that occurred in the German zone of occupation, and some German civilians were, according to Nicholas Stargardt in The German War, confused by the regime's quixotic public relations campaign: "Suddenly the 'Polish trash' who had massacred ethnic Germans at the outbreak of the war deserved their sympathy. This new-found solidarity only made sense, according to the SD [the German Sicherheitsdienst, "Security Service"], to those 'in intellectual and religious circles' who felt guilty on account of the 'far greater number of Poles and Jews eliminated by the German side.'" Nevertheless, the Katyn massacre created significant embarrassment for the Americans and British, whose overwhelming priority at that point in the war was the maintenance of their alliance with the Soviet Union, far above assuaging the grief and anger of their Polish allies.

Another irony was the extent to which Polish Jews were uniquely, if inadvertently, victimized by Soviet authorities. Polish citizens who refused to take on Soviet passports were subject to deportation into the USSR - they were, in effect, an undesirable bureaucratic obstacle to the thorough Sovietization of Polish society. Many of those who refused were Jews running from the Nazis who feared that taking a Soviet passport would sever their only connection to their homeland and prevent them from ever returning to an independent Poland. According to Snyder, "[o]f the 78,339 people deported in the June 1940 action that targeted refugees [from German-occupied western Poland], about eighty-four percent were Jewish." These were the first of many millions of Eastern Europeans to qualify as "double victims" - persecuted by the Nazis first, and then the Soviets in their turn.

Trying to compare the Soviet occupation tit-for-tat with the genocidal occupation of western Poland by the Germans during the same period is extraordinarily difficult. Comparisons would be hard enough, were it not for the fact that the Soviet occupation was followed by the German invasion of the Soviet Union and the Holocaust, the vast majority of which was executed on Polish soil in the years of German occupation between the Soviet retreat and return (1941-1944). We cannot help but allow our comparison to be affected by that context. And it seems like an actuarial betrayal of the humanity of the victims on both sides to engage in a dry comparison of numbers (many of which are, at best, rough estimates, anyway). But it would be morally and historically irresponsible to deny or trivialize those people who fell victim to the Soviet occupation. And, as Snyder claims, the Nazi and Soviet regimes in Poland murdered "in comparable numbers for similar reasons." For our purposes, it is perhaps enough to say that tens - perhaps hundreds - of thousands of innocent people were murdered by the Soviet authorities or died due to deliberate Soviet neglect, and many more were deported into lives of misery, all in the service of the "decapitation" and Sovietization of Polish society.