Onlookers pressed faces against windows. Drivers stopped cars. Employees exited their places of work to take in the spectacle. As simultaneous immigration-rights marches converged on Union Square yesterday, parts of Manhattan came to a standstill.

Chanting “Si se puede” and “We are American,” tens of thousands of immigrants and their supporters hit the streets in unison with demonstrations across the nation to protest against proposed legislation that would make illegally entering America a felony.

Hoisting his 1-year-old son, Joshua, in the air, Franco Baroga, 22, said he took the day off from his auto mechanic job in Yonkers to show people that immigrants, legal and illegal alike, deserve rights.

“We need protection, we need work. We need paperwork,” Mr. Baroga said.

Chinese and Polish immigrants joined a large Latino coalition at Sara Roosevelt Park on the Lower East Side. The group had no permit to march, but as their numbers grew to about 1,000, police relented and guided the march toward Union Square. Along the way, the marchers chanted, banged drums, and blew whistles.

Police presence was heavy throughout the city. Scores of scooter-mounted officers kept protesters from blocking streets and at least two surveillance helicopters hovered throughout the day. At Union Square, where protesters and flags pullulated over every foot of the park, police cordoned off most entrances. It took more than half an hour for some marchers to enter the park from a block away.

An auto mechanic from Brooklyn with no green card who gave his name only as William said new immigrants in America are routinely paid below minimum wage because they have no recourse to legal protection. After six years working at a bus repair shop, he said he was able to get a raise to about $7 an hour from $5. The income is barely enough for him to support his parents and brother back in Ecuador, he said.

“They treat immigrants like terrorists,” he said. “I hope the cause does something. They make it very hard for us.”

Fears that many immigrants would boycott work altogether and cause disruptions were assuaged early in the day. Stores along a section of 116th Street between Lexington and Second avenues were largely closed in solidarity with the protests, but closures at other locations in the city were sporadic. A restaurant on First Avenue and 10th Street, MomoFuku, closed for lunch because of the protests. A sign in the window read, “We will be closed for lunch Monday, May 1, to support the National Boycott for Immigrant Rights.” It reopened for dinner.

Leaders of the Break the Chains coalition said the issues involved in the disputed legislation aren’t limited to immigration.

“They are trying to create an easily exploitable underclass,” a representative for the National Mobilization Against Sweatshops, Michael Andrade, said. “This is not just about underpaid Latino workers, but workers across the community.”

The atmosphere among the protesters was festive. Even police officers seemed moved by the cause. An officer of the 5th Precinct was overheard confiding to a marcher: “If I weren’t in uniform, I would be marching with you guys. You guys are fighting a good cause.”

At 12:16 p.m., immigrant workers and supporters joined hands at eight locations around the city to protest the day, December 16, that the House of Representatives passed the bill that would make felons of illegal aliens.

A cutter for a dress factory in the garment district who participated in the human chain, Michael DiPalma, 43, said all New Yorkers should support the cause out of respect for their ancestry.

“Just about everybody in America was an immigrant once,” he said. “If you believe in this country, you should believe in immigrants.”

As the march from the Lower East Side pushed west on Houston Street, the owner of an antiques store, Billy LeRoy, watched with a demure expression.

“This is nothing like the May ’68 riots in France,” he said, smoking a thin cigar while sitting on a chair beside his Rottweiler, Killjoy. “I hope they get what they want, but it doesn’t look good.”