Military vehicles patrolled the streets before dawn with loudspeakers blaring, "We have photographs. We are going to make arrests!"
Shari Villarosa, the acting U.S. ambassador in Myanmar, said in a telephone interview with The Associated Press in Bangkok that people in Yangon were terrified.
"From what we understand, military police ... are traveling around the city in the middle of the night, going into homes and picking up people," she said.
Residents living near the Shwedagon Pagoda, Myanmar's most revered shrine and a flash point of unrest, reported that police swept through several dozen homes in the middle of the night, dragging away several men for questioning.
The homes were located above shops at a marketplace that caters to the nearby pagoda, selling monks robes and begging bowls.
The security forces were looking for people who had participated in demonstrations since mid-August, which troops brutally crushed on Sept. 26 and 27 with live gunfire, tear gas and baton charges.
The government says 10 people were killed but dissident groups put the toll at about 200. In addition, they say, some 6,000 people have already been arrested, including thousands of Buddhist monks who led the demonstrations, which initially started to protest a massive hike in fuel prices.
Villarosa said her embassy staff had gone to some monasteries in recent days and found them completely empty. Others were barricaded by the military and declared off-limits to outsiders.
"There is a significantly reduced number of monks on the streets. Where are the monks? What has happened to them?" she said.
On Wednesday, scores of monks were seen at Yangon's main train station. Witnesses said some of them were ordered by their superior monks to go back home to avoid trouble. Others were ordered by the government to vacate monasteries and head home to reduce the possibility of future unrest.
Some in Myanmar say the abbots are closely tied with the junta, while the younger monks are more sympathetic to the democracy protesters.
The Democratic Voice of Burma, a dissident radio station based in Norway, said authorities have released 90 of the 400 monks detained in Kachin state's capital, Myitkyina, during a midnight raid on monasteries on Sept. 25. Myanmar is also known as Burma.
Meanwhile, the junta pursued other means of intimidation. An employee from the Transport Ministry, speaking on condition of anonymity, said he was told to sign a statement saying he and his family would not take part in any political activity and would not listen to foreign radio reports.
Many Myanmar people use short-wave radios to pick up foreign English-language stations - a main source for news about their tightly controlled country.
On Wednesday, Human Rights Watch in Bangkok presented a man they said was a Myanmar army major who had fled the country to Thailand. The group released a transcript of an interview with the unidentified man in which he expressed shock at the crackdown.
"They (the demonstrators) were very peaceful. Later when I heard they were shot and killed and the armed forces used tear gas, I was really upset and I thought the army should stand for their own people," the man was quoted as saying.
Human Rights Watch declined to allow the AP to interview or photograph the man, saying it would compromise his safety.