BEIJING - Police kept Tiananmen Square free of demonstrators today, detaining at least 16 people while activists abroad marked the 15th anniversary of the deadly attack on pro-democracy protesters and pressed their demands for political change.

Since the June 4, 1989, military assault that killed hundreds, and possibly thousands, communist leaders have made many changes demanded by the dissidents, scrapping rules dictating where Chinese could work and whom they could marry. A decade of stunning economic growth has given millions new choices in life.

But the closed, secretive ruling party that crushed the protests still permits no independent political activity and has jailed or driven into exile most of China's active dissidents.

Reporters saw 16 middle-aged men and women picked up today on the square in twos and threes and dragged to waiting police vans. It wasn't clear whether the detentions were related to the anniversary, but security forces had been trying to block public commemorations for people killed in the military crackdown.

The square was open to the public and hundreds of tourists with their children were strolling under a light sprinkling of rain.

Though extra guards were on duty, security was relatively light compared with other politically sensitive dates. Troops from the paramilitary People's Armed Police dozed aboard two parked buses. Security agents in civilian clothes moved among the crowds.

An Associated Press photographer was briefly detained after photographing detentions on the square, and Chinese tourists who snapped pictures were forced by police to delete them from digital cameras.

In advance of the anniversary, Chinese authorities detained activists and relatives of people killed in 1989 or ordered them out of Beijing.

Broadcasts of CNN to hotels and apartment compounds for foreigners in the Chinese capital today were blacked out repeatedly when the network showed reports on the crackdown.

In contrast to the quiet in Beijing, veterans of the protests and other activists commemorated the deaths with vigils, marches and hunger strikes in Hong Kong, Washington and Taipei, Taiwan.

In Hong Kong, thousands of people were expected to attend a candlelight vigil today. The anniversary is especially emotional for the former British territory because Beijing recently ruled out full democracy there, stirring fears that it is losing the freedoms and autonomy promised when it returned to Chinese rule in 1997.

In Washington, a veteran of the demonstrations was in the midst of a fast outside the Chinese Embassy that began Tuesday.

"We should not just sit and wait for change. We've been waiting for 15 years and it hasn't happened," said Liu Junguo.