Is there a certain country/region you're interested in answers for? This seems a little general.
Edit: Since you're interested in everything, I'll just respond in this comment.
There's the Jallianwala Bagh massacre of April 13, 1919. This took place in British-ruled India during the growing independence movement, specifically in the city of Amritsar, a major city in the Punjab region, today part of northwest India/east Pakistan. The government of India said that 379 people died, and around 1,000 were injured. The Indian National Congress has higher estimates.
Background: The government of India passed the Rowlett Act in March of 1919. This was intended to help them deal with political terrorism/revolutionaries who wanted the British out of South Asia, and included things like trial without jury, arrests without a warrant, stricter control of the press, ect... This made a lot of Indians very angry. Gandhi decided to organize a nationally based satyagraha, in which all Indians would ideally participate, to protest against the Act. This was supposed to be non-violent, but things didn't go as planned. Four days after the satyagraha was set to begin, two local figures in Punjab, who supported Gandhi's satyagraha, and independence in general, were arrested by the government. While satyagraha is supposed to be non-violent, there was a lot of violence/unrest anyways. Once these leaders were arrested in Punjab, angry people attacked government buildings, cut telegraph lines, and disrupted the railway system. The situation in Punjab was especially bad, and the country was recovering from the 1918 flu pandemic and a recession. The government thought the situation was completely out of control, so they called in the army.
General Dyer took over and implemented martial law. Public gatherings of more than a few people were prohibited. A few days after the arrests I spoke of, thousands of people gathered in Jallianwalla Bagh to demand the release of these people. Lots of people from neighboring areas joined, since they were in Amritsar for a big festival that day. Jallianwalla Bagh was pretty well closed off, surrounded by buildings and houses on most sides. Without warning, Dyer blocked off the entrance with his troops and armored vehicles mounted with machine guns, and told his troops to start firing into the crowd. This lasted for about 10 minutes until they ran out of ammo. There were at least 15,000 people there, and it wasn't a violent demonstration. Aside from being killed by Dyer's forces, there were also a lot of deaths due to the crush to escape to safety.
Reaction: Though Dyer lost his position and was forced to retire to Britain, and some British people were extremely critical of him, many thought that he was correct to "punish" Indians through his actions, and treated him as a sort of hero who had upheld the law in Punjab. (Some names you may know: Winston Churchill was against Dyer, while Rudyard Kipling supported him.) The House of Lords supported Dyer's actions in a vote, and evidently enough of the public agreed that they were able to raise 30k pounds for Dyer upon his return to Britain. Indians were shocked, angered, and disgusted by the entire thing. A lot of them who had previously supported working with the British to achieve independence through "constitutional and legal means" lost confidence and interest in that approach completely. The Indian National Congress collectively shifted away from it with very little opposition. The reaction to this massacre indirectly gave the independence movement deeper and comprehensive support, so it was pretty much nation-wide, and reached into various social groups much more thoroughly than before.
Sources:
Rowlatt Acts - Encyclopedia Britannica
Essays on Gandhian Politics: The Rowlatt Satyagraha of 1919
India's Struggle for Independence - Bipan Chandra et al