Image caption Masked protesters - and police - were on the streets of Oakland throughout much of the night

Protests have continued throughout the night in Oakland, California, as police clashed with demonstrators from the Occupy Wall Street group.

Police fired tear gas as a group within the protesters lit a fire and threw stones and bottles at officers.

The late-night clashes followed a day of action against "corporate greed", which saw a mass protest march in central Oakland.

Protesters shut down the city's port, one of the busiest in the US.

Around 30 people were reportedly arrested in the early hours of Thursday, and at least four protesters were brought to hospital.

Protests on Wednesday were largely peaceful until around midnight local time (07:00 GMT), when some of the protesters reportedly lit a barricade on fire.

Afterwards, police fired tear gas and "flash bang" grenades near the main plaza, where Occupy Wall Street protesters have been camping.

Veterans protest

Hundreds of city workers, including teachers, in Oakland, joined the call for a strike.

The crowds also stopped traffic at a junction where a military veteran was seriously injured last week as protesters clashed with police.

Ex-Marine Scott Olsen, 24, is recovering in an Oakland hospital after being struck on the head with a tear gas canister fired by police.

That incident catapulted Oakland, which is on San Francisco Bay, to the centre of the national Occupy Wall Street movement and has spurred fresh demonstrations across the US.

Elsewhere in the US on Wednesday:

Police in Philadelphia arrested nine protesters who staged a sit-in inside the lobby of Comcast, America's largest cable firm

In New York, about 100 military veterans marched in uniform and stopped in front of the city's stock exchange, where there was a heavy police presence

In Boston, college students and union workers marched on Bank of America offices and the statehouse to protest over the student debt crisis

'Death to capitalism'

Organisers of the latest Occupy Oakland protest said they wanted to disrupt operations at the nation's fifth busiest port, which handles about $39bn (£24bn) a year in imports and exports.

"At this time, maritime operations remain effectively shut down," the Port of Oakland officials said at a news conference late on Wednesday.

Media playback is unsupported on your device Media caption The BBC's Alastair Leithead says the march on the port came during a day of protests

They said operations would resume "when it is safe to do so".

Oakland police estimated that about 3,000 demonstrators were gathered at the port. Police said 4,500 people had marched across the city on Wednesday.

But a spokesman for the protest movement, who only gave his name as Aaron, told the BBC: "It is an order of magnitude larger than any protests we've seen and we've seen some big ones in the last week."

The banks, the corporate greed and the people in government are letting the economy go down the toilet Nicole Warner, Protester

"There are pockets here and there going on for what seems like miles. I have heard people say 20,000, 30,000... it's impossible to tell."

But city officials later said that only about 150 protesters remained at the port late on Wednesday night and some 900 in the city's main square.

A number of businesses, including several banks, were shut during the protests, which were largely peaceful earlier in the day.

One protester, Nicole Warner, said the atmosphere had been good.

"My daughter and I were at the port all evening."

"It was the most beautiful sight I'd ever seen, thousands upon thousands of people piling over the bridge in a totally peaceful way."

Explaining why she had joined the demonstration she said: "People are losing their homes and they don't have jobs."

"The banks, the corporate greed and the people in government are letting the economy go down the toilet."

City officials stressed that there were isolated incidents of violence led by groups of anarchists among the protesters.

Some Occupy Oakland protesters returned to scrub the walls of a Wells Fargo bank after they had been defaced with graffiti, the Los Angeles times reported.

Oakland Mayor Jean Quan, who had been criticised for her handling of earlier protests, called for calm.

Image caption City officials blamed anarchists for "isolated" incidents of violence

The demonstration, which included students, families with young children and union members, began with a rally outside city hall.

One large protest banner read: "Occupy Everything, death to capitalism."

Oakland officials said about 5% of employees had taken the day off, including about 360 teachers.

One teacher at the rally, Brother Muziki, told AFP news agency: "Our classrooms are overcrowded. The banks are being bailed out - but not the schools."

The BBC's Alastair Leithead in Los Angeles says Oakland has a higher than average unemployment rate and suffered badly during the US recession.

The activists have been protesting against a system which they believe favours corporations and the nation's wealthiest 1%.