According to Mr. Nielsen, authorities at the Nakivale refugee camp, where nearly 10,000 refugees from Rwanda live, told asylum seekers to assemble at certain places to receive free food.

When the refugees realized they had been tricked, Mr. Nielsen said, they began to panic. Ugandan police officers fired their weapons into the air. At least 25 people were injured trying to escape, and families became separated, leaving small children alone without their parents.

“What really worried us was the way in which it was done,” Mr. Nielsen said. “The refugees were lured to collection points under false pretenses.”

Witnesses told local journalists near Nakivale that four people had died, some shot by the police.

Musa Ecweru, a Ugandan government minister, told local journalists that the operation had focused on illegal immigrants who had already been refused asylum.

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Refugee laws are relatively lax in Uganda, which was attacked on Sunday by the Shabab, a Somali militant group that killed 76 people in coordinated bombings in the capital, Kampala. Mr. Nielsen said Uganda normally accepted all Somali asylum seekers, but only about 2 percent of Rwandan refugees were accepted.

“There are no problems with Congolese, Sudanese, Somalis, Kenyans,” Mr. Nielsen said. “But they have this particular bilateral agreement with Rwanda.”

Since the end of genocide in 1994, Rwanda has sought to bring all refugees home. The forced repatriation this week was not the first time such measures had been used.

Last year, roughly 400 Rwandans — many of them believed to be ethnic Hutus — fled the country to neighboring Burundi. The refugees, who said they feared for their personal safety in Rwanda, were later lured into trucks by the Burundian police with promises of food and sent back to Rwanda, much like in Uganda this week.

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Rwanda’s government has said many of the refugees are actually fugitives who committed crimes during the genocide.

“Did you consider the fact that some refugees are running away from accountability?” asked Maj. Gen. Frank Mugambagye, referring to a report on the Rwandan refugees by local watchdog groups. “Rwanda is the only country in the world where most of its refugees have returned home.”

In August, Rwanda will hold its second presidential election since 1994.

Since the beginning of the year, deadly grenade attacks have struck the Rwandan capital, Kigali; newspapers have been closed; and journalists, politicians and senior military officers have been arrested. The United Nations said Uganda received a surge of Rwandan asylum seekers earlier this year.