World Economic Forum protesters are being given fair warning: Wear a mask, and city officials will give you a costume — a prison jumpsuit — to match.

Chief of Patrol Joseph Esposito said Monday that New York intends to enforce a law that bans groups of people from wearing masks in public.

"Three or more with masks and they're marching, they're under arrest," Esposito said.

Thousands of demonstrators are expected to be on the streets of New York starting Thursday, when political and business leaders from around the world begin arriving for the four-day event.

Protest organizers say many marchers plan to don costumes and carry giant puppets — some worn over their heads — to emphasize their anti-globalization message.

"They're going to have to arrest thousands and thousands of people," said David Graeber of the Anti-Capitalist Convergence, an anarchist group. "It's not going to be good."

Nearly 4,000 police officers are expected to be near the Waldorf-Astoria hotel in midtown Manhattan. Hundreds more will protect foreign dignitaries, including Afghan interim leader Hamid Karzai, and visiting chief executives like Microsoft chief Bill Gates.

The World Economic Forum, which is usually set at a swanky resort in Davos, Switzerland, is privately funded and is billed as a gathering of 3,200 of the world's top business leaders, economic experts, academic and government leaders to discuss the state of world economics.

Organizers moved this year's conference from Davos, partly to show solidarity with New York City after the Sept. 11 attacks.

But anti-globalization activists are vowing to continue their often-violent crusade against what they see as corporate injustice at the scene of the crime: New York City.

Activists involved in plotting the New York protests are the same who led the raucous and violent December 2000 demonstrations at a World Trade Organization meeting in Seattle, as well as equally violent demonstrations in Washington, Prague and Genoa. In Seattle alone, some 500 people were arrested, protestors destroyed dozens of businesses and a civil emergency was declared to restore order.

John Cavanagh, the director of the Institute for Policy Studies, which is joining other groups to organize rallies and teach-ins next week, said the groups he is working with have pledged peaceful protests in New York.

But earlier pledges of peace at such meetings have not held. The protests were taken over by more radical elements of the anti-globalization movement such as the Ruckus Society and freelance anarchist groups. A spokesman for the Ruckus Society refused comment to Fox News about its plans for New York, citing an "embargo" against the news network.

A new component of the New York protest scene will be a strong anti-war contingent, though not all the participants are expected to embrace that mantra. Thea Lee, a public policy director for the AFL-CIO, which is sending a contingent to speak inside and outside the forum, said her group would not be taking part in the anti-war protests.

"We are sensitive to those issues," Lee said. "We want to do this as non-violently as possible."

War won't be the only thing activists will be bringing to the table. Expected to attend are Students for Global Justice, The Public Eye on Davos, Reclaim the Streets, The International Action Center, Public Citizen, Workers Democracy Network, International Socialist Organization and the anarchistic Anti-Capitalist Convergence.

Organizers would not estimate the numbers expected in New York next week or whether it would be free of the spectacle that drew so much media attention at previous demonstrations. "It's hard to say, there will be anywhere from thousands to tens of thousands," Cooper said. "There are going to be activists coming in from all over the world."

Fox News' Kelly Beaucar Vlahos and The Associated Press contributed to this report.