Police said 15,000 people marched peacefully from Hyde Park through Picadilly Circus and later packed Trafalgar Square under a gray autumn sky. The Stop the War Coalition, which organized the march, put the turnout at 50,000, including people from anti-nuclear, anti-capitalist and Muslim groups.

Participants said that despite a week of major developments in Afghanistan, where the anti-Taliban northern alliance has won a string of decisive victories, many Britons still opposed the use of force to fight terrorism.

"Killing the people in Afghanistan does not bring back the lives in New York City," said Paul Embery, a London firefighter. "Fighting is just fanning the fires of terrorism."

As marchers passed through Picadilly Circus, a group of protesters carrying a banner saying "Muslims for Justice" briefly sat down in the street.

"People are just angry and they want to voice their concerns," said Jawid Yakoob, an accountant who traveled from the central English city of Birmingham to join the Muslim group.

Later, demonstrators congregated in Trafalgar Square, where politicians and activists addressed the crowd into the evening hours.

"We have seen the removal of one feudal tyranny, only for it to be replaced by another," said Alan Simpson, a Labor Party lawmaker, referring to the northern alliance, which now controls parts of Afghanistan previously under Taliban rule.

As the sun set, thousands of demonstrators – both Muslims and non-Muslims – shared dates and water to celebrate the end of a day of fasting as part of the holy Muslim month of Ramadan.