By Laird Harrison

OAKLAND (Reuters) - Police fired what appeared to be smoke grenades on Saturday at a group of hundreds of Occupy Oakland protesters who tore down a chain-link fence as they tried to gain entrance to the city's shuttered convention center.

The scuffles marked the latest confrontation between police and Occupy protesters seeking to regain lost momentum in their movement against economic inequality after authorities cleared protest camps around the country late last year.

Occupy Oakland organizers had vowed to take over the abandoned building to establish a new headquarters for their movement and draw attention to homelessness in a move seen as a challenge to authorities who have blocked similar efforts before.

But near the convention center, several dozen police officers declared an unlawful assembly and confronted the demonstrators at the fence, firing smoke canisters into the crowd after telling them to disperse through loudspeakers.

The crowd fell back as the smoke hit, but then made a second push toward the fence, where they were held back by police. Some crowd members tried to circumvent the police line, and surged toward police as more smoke canisters were fired, carrying shields made of plastic garbage cans and corrugated metal.

"The City of Oakland welcomes peaceful forms of assembly and freedom of speech, but acts of violence, property destruction and overnight lodging will not be tolerated," police said in a statement.

Police said they had yet to use tear gas, but made no mention of smoke canisters. They said "use of gas may occur if necessary."

Protesters in Oakland loosely affiliated with the Occupy Wall Street movement that began in New York last year have repeatedly clashed with police during a series of marches and demonstrations.

In October, former Marine Scott Olsen was left in critical condition with a head injury following a confrontation with police on the streets of Oakland in which tear gas was deployed.

Organizers say Olsen was struck in the head by a tear gas canister. Authorities opened an investigation into that incident but have not said how they believe he was hurt.

(Reporting by Laird Harrison; Writing by Dan Whitcomb and Mary Slosson; Editing by Cynthia Johnston)