"I think the vast majority of people who protest were peaceful," said Police Commissioner Raymond Kelly.
"But there's clearly a core group of self-styled anarchists - that's what they call themselves - who want to have a confrontation with police."
He said that following Wednesday's 10,000-strong union march, a much smaller group tried to storm police barricades at Wall Street and Broadway.
"They locked their arms. They counted down - 10, 9, 8, 7, 6. Then they decided to charge the police. That is going to be met with some physical force," Kelly said.
Occupy Wall Street - the leaderless anti-corporate movement that has spread to 150 cities - said it was the police who were eager to provoke violence.
The group posted a YouTube video of a cop a block from the Stock Exchange saying Wednesday evening that he just couldn't wait to beat up demonstrators.
"My little nightstick's gonna get a workout tonight," the burly officer whose badge reads 'Rodriguez' says gleefully to a fellow cop as they wait by a metal barricade on Broad Street.
That clip was juxtaposed by protesters with video taken in the melee later on that shows a police lieutenant holding a baton in both hands and wading through a crowd indiscriminately clubbing people.
"Police officers have a right to use force and to defend themselves when they are being charged by a group trying to overwhelm them," said NYPD chief spokesman Paul Browne.
No injuries were reported, he said.
Twenty three people were arrested. Most were charged with disorderly conduct but one man who police said knocked a cop off a scooter was charged with assault.
"There just is a standard of conduct which is a line you cannot cross," he said.
As to cops bragging about using their nightsticks, the mayor said he couldn't account for every officer.
"Our Police Department conducted themselves the way that they should. Every cop? I don't know. There will always be somebody that has some piece of footage," Bloomberg said.
Police cannot legally boot the protesters from the downtown's Zuccotti Park, a public space.
But Assembly Speaker Sheldon Silver said Wednesday the protesters should quit blocking one of the area's few parks, saying residents had suffered enough blocked streets during 9/11.
Some downtown residents have complained that drumming and chanting in the park is keeping them awake.
The group plans to take over Washington Square Park on Saturday.