Witnesses said at least eight truckloads of prisoners were taken from central Yangon, the former Burma's biggest city, where crowds of up to 100,000 people had protested against decades of military rule and deepening economic hardship.
A staff member of the U.N. Development Fund, her husband, brother-in-law and driver were among those arrested, U.N. spokeswoman Michele Montas said in New York. The United Nations was appealing to Myanmar's U.N. mission to secure her release.
The crackdown continued despite some hopes of progress by U.N. envoy Ibrahim Gambari on his mission to persuade junta chief Than Shwe to relax his grip and open talks with detained opposition leader Aung San Suu Kyi, whom Gambari met twice.
U.N. Secretary-General Ban Ki-moon said he would meet Gambari on Thursday, then on Friday discuss with the 15 members of the Security Council how to address human rights violations in the Southeast Asian country.
"That is one of the top concerns of the international community," Ban said. Asked about Gambari's four-day mission, Ban replied: "You cannot call it always a success."
China, the closest the junta has to a friend, has made rare public calls for restraint but rules out supporting any U.N. sanctions against Myanmar. Russia, like China a veto-wielding member of the Security Council, also opposes sanctions.
Singapore, the current chair of the Association of South East Asian Nations (ASEAN), said it "was encouraged by the access and cooperation" given by the junta to Gambari.
The envoy was in Singapore on his way back to New York but is likely to say nothing in public before he briefs Ban.
U.S. first lady Laura Bush, who has been an outspoken supporter of human rights in Myanmar, called on the junta to step aside.
"The United States believes it is time for General Than Shwe and the junta to step aside, and to make way for a unified Burma governed by legitimate leaders," she said in a letter to the U.S. Senate foreign relations committee on Wednesday.
The protests -- the biggest challenge to the junta since it killed an estimated 3,000 people while crushing an uprising in 1988 -- began with small marches against fuel price rises in August and swelled after troops fired over the heads of monks.
Gambari had been "assured" of another visit to Myanmar in November, Ban said. But there were no signs how his mission and international pressure might change the policies of a junta which seldom heeds outside pressure, has endured years of sanctions by Western governments and rarely admits U.N. officials.