The Uzbek government has detained and questioned hundreds of people in a campaign of intimidation aimed at securing testimony that backs its version of events about the killing of demonstrators in the city of Andijan in May, according to a new report by Human Rights Watch.

"Police and security agents threatened or severely beat many of those detained in order to coerce them to confess to belonging to extremist religious organizations and bearing arms while participating in the May 13 protest," the New York-based advocacy organization said in an 81-page report titled "Burying the Truth: Uzbekistan Rewrites the Story of the Andijan Massacre."

The first 15 people charged with organizing an armed revolt in Andijan were to go on trial Tuesday in the Uzbek capital, Tashkent. Prosecutors allege that the 15 were part of a broad conspiracy, financed from abroad, that was designed to overthrow the government and replace it with an Islamic state. Prosecutors said another 106 people may face similar charges.

The deputy chief prosecutor, Anvar Nabiyev, said last week that "external destructive forces" had given organizers of the revolt $300,000. He said two-thirds of it came from the Islamic Movement of Uzbekistan, a group listed as a terrorist organization by the U.S. government.

Human rights groups charge that government forces killed hundreds of unarmed civilians in Andijan after a day of protests that were largely driven by anger over poverty and government repression. The protests began when armed men freed 23 prominent businessmen from a local prison early on May 13; the men were on trial for alleged membership in a banned Islamic group.

The mass killing led to international demands for an independent investigation, but the Uzbek authorities rejected those calls, leading relations with the United States, among other Western countries, to sour. Uzbekistan ordered the U.S. government to remove its forces from an airfield the Pentagon was using to support operations in Afghanistan.

After the mass killing, Andijan was effectively closed to outsiders, and locals were warned not to talk about the incident. Many people were visited at home by neighborhood committees. The government announced that 187 people were killed and 287 wounded in the violence, and it said most of them were terrorists.

"It was in this atmosphere of fear that the authorities detained hundreds -- and perhaps thousands -- of people in Andijan with the purported aim of obtaining testimony about the crimes committed on May 13, as the government has defined them," Human Rights Watch said.

The government has also arrested human rights workers, journalists and political activists who attempted to investigate the killings. At least 11 activists have been detained and another 15 have been forced into exile, according to Human Rights Watch. One long-time activist, Saidjahon Zainbitdinov, who was widely quoted after the killings, has been in prison since May 21 on charges of terrorism and sowing panic among the population.

In Tashkent and other cities, human rights activists have been the target of "hate rallies and other public denunciations in which local community leaders vilify them, calling them Islamic extremists and enemies of the people, and mobs attempt to run them out of town," according to Human Rights Watch.

The organization called on the United States to freeze any remaining military and counterterrorism assistance to the Uzbek military and said the European Union should suspend assistance to the country. The United States and the E.U. should also stop issuing visas to senior members of the Uzbek government, the group said.