BEIJING, MAY 31 -- Three young political activists who have been the Communist government's only open critics since the student-led democracy movement was crushed by Chinese troops last year dropped out of sight today, and the news conference they had planned with foreign journalists was canceled.

It was not clear if the three men were voluntarily lying low or had been taken into custody by authorities as a cautionary move before Sunday's first anniversary of the military attack on democracy demonstrators in Beijing that left hundreds dead. The three men had planned to issue an open letter to Chinese leaders calling for the release of political prisoners.

Police in Beijing and other cities have tightened security in the weeks leading up to the anniversary, and the official media have reported stepped-up arrests of vagrants and petty criminals, apparently to help ensure order.

Friends said two of the three missing activists, well-known folk singer Hou Dejian and former university newspaper editor Gao Xin, both 34, had not been seen since they left Hou's home early today. Several hours later, Hou phoned home to say that he was well but would not be back for an unspecified time. Gao phoned later with a similar message.

Hou also phoned two foreign journalists and said, "Because I personally have things to do, I'm not ready for a news conference." He refused to elaborate. The third man, Zhou Duo, 43, also has not been seen.

Foreign Ministry spokeswoman Li Jinhua said at the ministry's weekly briefing later today that Chinese were free to talk to foreign journalists but that a "certain kind of formalities" should be observed for interviews. She did not comment directly on the three missing dissidents.

Their disappearance came within hours after the government announced the executions of 20 people, including a man convicted of setting fire to a military jeep during the democracy demonstrations last year.

The arson suspect, identified as Yu Yongchuan, and the 19 others -- convicted of murder, robbery, rape and "hooliganism" -- were shot dead immediately after trial, according to the official Beijing Daily. The paper also reported the executions of 11 criminals the day before and noted that the court had cited the need to ensure social stability for the Asian Games in Beijing this September.

The official press has reported the executions of at least 20 people on charges stemming from last year's demonstrations, but the actual number is believed to be much higher.

Had the three missing activists succeeded in criticizing the government in an open news conference, it would have been an act of remarkable boldness in the current climate of tight political control. They and a fourth man, Liu Xiaobo, staged a hunger strike in Beijing's Tiananmen Square last June 2 to encourage student demonstrators camped there to demand democratic reform. When soldiers surrounded the square the night of June 3, they negotiated a peaceful retreat for the students.

All but Hou were arrested soon after. He took refuge in the Australian Embassy for more than two months, emerging only after the Beijing government promised he would not be arrested. Gao was released in December, and Zhou on May 10. Liu remains in prison without charge.

Over the past few months, Hou, Gao and Zhou gave many interviews to foreign journalists, and all three continued to call for peaceful transition to democracy. No other activists have been willing to be quoted by name since last year's crackdown.