WASHINGTON (AFP) - A top leader of the Tiananmen Square protests said he found that the movement's message resonated more than ever after he slipped into China to mark the 25th anniversary quietly.

Zhou Fengsuo had been number five on the Chinese government's wanted list as it crushed the student-led pro-democracy protests on June 3-4, 1989. Like most leaders at Tiananmen Square, he lives in exile with no prospect of returning back legally.

To his own surprise, Zhou said that he managed to return briefly for last week's anniversary of the crackdown even though China took extraordinary measures to prevent public observances. Now a US citizen, Zhou said he took advantage of China's policy of allowing 72-hour transit stops without a visa.

Zhou said he commemorated the bloodshed by driving with a friend in a loop around Tiananmen Square where he said he saw at least 10 groups of police. He resisted the temptation to carry out a public protest, knowing he would be quickly muzzled.

"If I couldn't hold back my emotion, I may have just jumped out of the car to shout, but then I would be gone in a minute," Zhou told AFP by Skype on Monday after his return to San Francisco.

But Zhou said he saw small signs of mourning. At Tsinghua University, where Zhou studied physics, he snapped a picture of white flowers laid at what had been a monument to the Tiananmen dead. Inside the square itself, Zhou saw people dressed in black in what he interpreted as a protest.

China's leaders have tried to stamp out memories of the uprising, with many young people unfamiliar with the mass movement. Troops killed hundreds of unarmed civilians, with some estimates putting the death toll at more than 1,000.

But Zhou said that he believed the core principles of the students rang true - such as demands that government officials disclose their assets, an issue that has triggered small-scale demonstrations and arrests this year.

"There is pretty much a consensus today in China that government officials should disclose their assets," he said.

The past 25 years have proven "that the massacre was for this small fraction of families that control China, not for the general well-being of the Chinese. If you look at the people who were hardliners 25 years ago, they are all billionaires now," he said.

Zhou said that he also showed solidarity by visiting a detention centre where Gao Yu, a veteran reporter previously jailed for writing about the Tiananmen protests, and celebrated human rights lawyer Pu Zhiqiang were being held.

Zhou said he identified himself at the reception and faced no problem. But later police came to his hotel on the pretext of searching for drugs and then interrogated him at length and put him on a flight back to the United States, Zhou said.

The activist, who has worked in finance since going into exile, said he told no one of his trip ahead of time. Fellow Tiananmen leader Wang Dan even spotted, during an event in Washington, that Zhou was carrying Chinese currency and joked that he was being paid by the embassy.

Zhou revealed that he had also visited China secretly on two other occasions since the Tiananmen crackdown. But he acknowledged that a future trip would prove more difficult after his latest episode.

"For me, I have overcome my doubt and fear to be there with my friends in China. That is tremendous for me," he said.