Banners proclaimed: "Islam will dominate the world" and "Those who do not rule by what Allah has revealed are oppressors." Preachers Abu Hamza al-Masri and Omar Bakri Muhammad addressed hundreds, casting Islam as the salve to a U.S.-led world of suffering.
British authorities allowed the rally - after police confiscated gasoline from beneath a podium - as free speech.
The scene showed how Islamist leaders who made London a base for global jihad, or holy war, before Sept. 11 are able to continue today. Bakri has called Trafalgar Square his new Mecca.
Osama bin Laden had an office in London. Preachers Hamza, Bakri and others at mosques taught hundreds of followers who later went to Pakistan and Afghanistan.
Followers included James Ujaama, 36, detained in Denver on July 22 and held by the FBI as a witness for a federal grand jury. Young men wearing camouflage combat jackets over Middle Eastern-style tunics still gather at Hamza's red-brick north London mosque, browsing videos and books featuring bin Laden. Hamza said he enlists eight new followers a month.
Accused would-be shoe-bomber Richard Reid hung out here. Suspected 20th hijacker Zacarias Moussaoui, the only man charged in connection with the Sept. 11 attacks, reportedly spent time in London.
Hamza, 44, lost both hands and an eye in a land mine explosion during the early 1990s in Afghanistan.
The enemy isn't American people - only American policies, he said.
"The American people, they are not getting what they deserve from their politicians. They are getting hatred, loss of security."
He defined jihad as "clean war," which he contrasted with wars waged by U.S. soldiers.
"Like a surgery," he said of holy war. "You take out the bad thing without interrupting anything else."
Hamza said he has no links with Osama bin Laden. He scoffed at reports that he may have had a role setting up a suspected jihad training camp in Oregon. FBI agents may well indict him anyway, he said.
"Yes, it will come," Hamza said. "It will be fun to show their ignorance, if they don't keep it secret as usual."
U.S. authorities holding Ujaama and about 200 others as part of the investigation into the Sept. 11 attacks have tried to extradite Islamist leaders from Britain. But British courts recently have blocked U.S. requests. And a public rally such as the one Sunday is tolerated.
"It's a balance, isn't it?" said David Veness, chief of counter-terrorism and special operations for London police. "The expression of opinion, views - whatever our personal opinion - is a matter for the laws of the land and the rights enshrined in those laws."
Certainly free societies face risks, Veness said: "One has to acknowledge the fact that rhetoric can lead to incitement of violence. One has to be alert to that."
Yet in Britain and internationally, he said, counterterrorism now should be "focused on those who are actively engaged in causing actual people harm by violent means."
A British government official dealing with counterterrorism, speaking on condition of anonymity, said officials "are fully aware of this individual (Bakri) and his organization. . . . Their activities are closely monitored. Anyone breaking the law and inciting violence . . . will be prosecuted. If there were any evidence at all to suggest that any individual was actively involved in terrorism, they would be arrested."
Britain's 2.5 million Muslims for the most part espouse more moderate politics than Bakri and Hamza, said Mohammad Sawalha, president of the Muslim Association of Britain. Hamza, Sawalha said, is an Egyptian-born imam wanted in Yemen for alleged involvement in attacks there.
"In this country, he is allowed to say whatever he wants. But he isn't doing anything," Sawalha said.
At Sunday's rally, the preachers, called imams in Islam, and several lieutenants addressed a crowd estimated at 600 from microphones at the base of the towering stone column at Trafalgar Square - a monument to British military power. They spoke for two hours. Views presented were primarily peaceful, a call to Islam.
Yet right before he spoke, Sulayman Abu Ibraheem, 31, from a London suburb near Gatwick airport, decried what they called American injustice toward Muslims. He said he hadn't fought in Pakistan or Afghanistan - yet.
"Unfortunately, I haven't had that honor," Ibraheem said.
Police arrested him after his public address, citing what they considered an inflammatory use of the words "bin Laden."
Other followers wearing black headbands stood at booths with pamphlets and videocasettes presenting Osama bin Laden with titles such as "Islam, Terrorism and The New World Order."
Right-wing dissenters waving British flags tried to break up the rally, kicking over loudspeakers. Among them was lighting shop owner John Sawyer, 54, who said the rally "is an insult" in light of Sept. 11.
Police from a force of about 150 made four arrests and prevented major fights.
"Makes me sick," said an American tourist at the rally, casino manager Mike Couevas, 52, from Las Vegas. "Can they do this?"
Los Angeles artist Louis Braatz, 27, who said he visits Colorado regularly with his parents, staked out the middle of the square holding up a small, tattered United States flag.
Denver-born minister Richard Eachus, 67, now living in Seattle, sat on a wall and tried to make sense of it all.
"I do believe in free speech," he said. "But what they did to America, they forfeited that right. The line is thin between what they are doing now and what happened on Sept. 11. Would Britain have allowed Hitler to hold a rally here in 1939? Of course not."