Downtown Berkeley became a battleground of its own kind today as 500 anti-war and pro-military protesters faced off in a public plaza over the Marine Corps' recruiting center in this famously liberal city.

The City Council is scheduled to decide tonight whether to revoke a letter it approved two weeks ago telling the Marines they are "unwelcome intruders" and should leave. The council's action provoked widespread debate.

[Video: Anti-war and pro-military protesters face off at dawn.]

In anticipation of the meeting tonight at City Hall, anti-war protesters lined up on one side of Martin Luther King Jr. Way while pro-military groups took control of the other side.

Things were peaceful for most of the morning - save for an argument between skateboarders upset that the pro-military side had taken over their skate park.

But around noon, things heated up as more than three dozen police with batons and riot gear formed a line to separate the two sides. The police intervention occurred after several dozen demonstrators who want the recruiting center to leave town crossed the street and engaged in a 15-minute shouting match with a group supporting the Marines. There were no arrests.

Protesters on both sides of the police line amplified their messages: Recruiting opponents, using megaphones, chanted: "Murder, rape, torture, war. That's what the Marines are for," while military backers played the Marines' Hymn over loudspeakers.

About 250 people showed up on each side. They included dozens of students from nearby Berkeley High School, who chanted "One, two, three, four. Berkeley High against the war."

Others cane from outside the city. One of them, Mary Mankowski of Portland, Ore., said she paid $350 to fly in Monday.

"It kills me to pay full fare but this is important," said Mankowski, who claimed that every generation of her family has served in the military since the American Revolution. "(This is an) outrageous erosion of our constitutional privileges."

Diane Britto of Lafayette and her friend Elynne Allen of Pleasant Hill vowed they would never spend another dollar in the city of Berkeley. Britto, whose son is in the Navy, said she would keep her season tickets to Cal football games but will have dinner in Oakland on game days from now on.

Berkeley's decision, she said, was "despicable to the rest of the nation."

Allen, whose husband served in the Marine Corps in Vietnam, said she would push her car outside the city limits if it runs out of gas while she's in Berkeley.

Anti-war protesters gathered on the other side of the street, including members of the group Code Pink, which has held weekly protests in front of the recruiting station on nearby Shattuck Square. They sold T-shirts, waved flags and sang "We Shall Not be Moved" through a loudspeaker.

"I'm here to thank the city council for dis-inviting the Marines," said Berkeley resident Tim Modok, who was attending with his schnauzer, Susie. "I'd rather have a porn (outlet) two blocks from an elementary school and a high school than I would a Marines recruiter; they're telling kids lies to get them into this war. These are very dangerous people. They're lying to their children, talking them into becoming killers."

Linci Comy of Oakland said she was there for future generations.

"I don't want my kids and grandkids to go to another war of occupation," she said. "We have to set a standard, we have to tell the world that military recruitment is no longer acceptable."

As the protests raged, the Marines recruiting station was closed and a lone police officer on a bicycle patrolled outside.

The City Council meets at 7 p.m. tonight at Maudelle Shirek City Hall, 2134 Martin Luther King Jr. Way, Berkeley. An overflow crowd is expected for discussion of the Marines recruiting center, which is expected to begin about 9 p.m. For those who don't get inside, the city will broadcast audio from the meeting from loudspeakers outside City Hall.

E-mail the writers at srubenstein@sfchronicle.com and mlagos@sfchornicle.com.