(AP) -- Chanting "I can't breathe" and carrying signs saying "black lives matter," tens of thousands of people marched up New York's Fifth Avenue and down Broadway on Saturday as part of a day of protest centered in the nation's capital over the deaths of unarmed black men by police.

Saturday's outpouring of demonstrators followed a series of protests over the last several weeks since grand juries decided against bringing charges against white police officers in the shooting death of Michael Brown in Ferguson, Missouri, and chokehold death of Eric Garner in Staten Island.

The noisy march through the heart of Manhattan swelled to at least 25,000 people, police said. It remained mostly peaceful until violence broke out as protesters marched across the Brooklyn Bridge, blocking traffic.

NYPD Chief of Department James O'Neil said two lieutenants were injured by protesters as the officers tried to arrest a man who had attempted to throw a garbage can from the bridge onto other officers below. No arrests had been made late Saturday. The man was able to flee, O'Neil said.

The officers were treated for bumps and bruises, including a broken nose.

Deputy Commissioner John Miller said officers recovered a backpack, which they believe belongs to the man, containing three hammers and a black face mask.

In a separate incident on the bridge, a person was arrested for disorderly conduct, and earlier Saturday the rear window of an NYPD traffic agent's car was smashed, police said.

Mayor Bill de Blasio condemned the violence, saying in a statement it "marks an ugly and unacceptable departure from the demonstrations thus far."

Members of the Brown and Garner families participated in a march in Washington on Saturday. They joined the Rev. Al Sharpton and thousands of others who marched down Pennsylvania Avenue.

In New York, Rich Alexandro, 47, carried a handmade sign with dozens of names of people who died in encounters with police who were never charged.

"It just seems like the cops are Teflon," Alexandro said. "There's no justice."

Police in Garner's death say he was resisting arrest and was put in a legal arm-hold before he fell to the ground and began complaining "I can't breathe." The officer in Brown's death said that Brown was combative and tried to get his gun; he said he feared for his life when he shot the 18-year-old teenager who some witnesses say was raising his hands apparently in response to the officer's commands.

Donna Carter, 54, marched with her boyfriend, whose teenage son was shot and killed by police in the 1990s while carrying a toy gun.

"It's good to see people of all colors here to say enough is enough," said Carter, who is black. "I'm a parent and every child that's killed feels like my child."

Family members of people killed in New York City police encounters going back decades also were among the demonstrators. They included Iris Baez, whose son Anthony Baez died after he apparently was placed in a chokehold in 1994.

Protesters started in Washington Square Park in Greenwich Village and headed toward midtown. They circled back downtown and ended the march at police headquarters in lower Manhattan where they were kept at a distance from the entrance by barricades and hundreds of police officers.