Pro-democracy activists in Myanmar say that more than 20 people have been arrested in the largest crackdown on dissent since the introduction of political reforms last year.
They were detained ahead of a commemoration of the suppression of a student movement 50 years ago.
About 300 people gathered in Yangon to mark the event, despite the detentions.
Two government officials have confirmed the arrests.
Myanmar appoints US ambassador
Meanwhile, Myanmar has approved the appointment of its first United States ambassador in 22 years.
State media say he is Derek Mitchell, 47, an expert on Asian affairs who is President Barack Obama's special representative and policy co-ordinator for Myanmar.
His appointment marks a significant upgrading of diplomatic relations between the two countries, which have been on the mend since political changes in Myanmar.
The country held its first general election in two decades in November 2010, bringing to power a government led by the pro-military Union Solidarity and Development Party.
President Thein Sein has initiated a host of political and economic reforms, including a dialogue with opposition leader Aung San Suu Kyi that paved the way for her to enter mainstream politics.
Ms Suu Kyi and 42 members of her National League for Democracy party won seats in a by-election in April.
Shortly after the by-election both the United States and the European Union eased economic sanctions that had been imposed after the army cracked down on a pro-democracy movement in 1988, which left an estimated 3,000 people dead.
