Hundreds of demonstrators threw rocks and fired slingshots at police, who answered with pepper spray, rubber bullets and finally a charge on horseback. At least 10 people were arrested and four injured.
The altercation began in an area designated for protests near the convention hall at the Staples Center as thousands began to leave after a concert by the rock group Rage Against the Machine.
Speeches by First Lady Hillary Rodham Clinton and then President Clinton continued inside with delegates generally unaware of the trouble outside. Police herded the protesters to the far end of the protest area and away from the convention hall.
"Today, tomorrow or the next day, or the next day, or the next week, our response will be exactly the same," Cmdr. David Kalish, an L.A.P.D. spokesman, told reporters gathered on a downtown street littered with chunks of asphalt and concrete, sticks and smashed plastic water bottles.
CBSNews.com's Susan Walsh reports that mounted police clearing a parking lot near the convention hall pushed a group of protesters and reporters into a fence. Despite the crush, there were no serious injuries.
The push to clear the area came after a running battle between police and protesters along a fence directly across from the convention.
The confrontation involved about 300 people out of the crowd of 8,000, but then spread as more officers rushed to the scene.
Protesters, most of them with bandannas over their faces, lit several fires in the concert area. The ground around the officers was littered with rocks, broken bottles, sticks thrown over the fence from the crowd.
The Rev. Jesse Jackson and the American Civil Liberties Union of Southern California accused the police of overreacting.
"Unnecessary brutality," Jackson called it.
The L.A.P.D.'s Kalish disagreed.
"Some will view it as we waited too long. Some will view it as we moved in too quickly," he said. But he insisted the action had been "a measured, strategic response."
Rage Against The Machine, known for its hard-driving sound, is anti-authoritarian, fiercely committed to leftist politics and has campaigned in support of such figures as Mumia Abu-Jamal, a former radio reporter on death row for killing a Philadelphia police officer in 1981.
The concert lasted about an hour, capping a day of protests at the convention's opening.
Earlier Monday, hundreds of demonstrators marched on the convention hall to protest Al Gore's ties to Big Oil. But, as CBSNews.com's Pete Brush reports, the talk of the crowd then talked of the ballot box rather than the barricades.
While the protesters focused tough criticism on the Democrats, their hope was to influence the convention rather than to disrupt it. Activists told CBSNews.com that blockin or stopping the Democratic gathering was not one of their objectives.
Nine arrests were reported Monday afternoon as demonstrators sat in an intersection about a half-mile from the Staples Center, where the convention is being held. But things were nowhere near as intense as the disruptions faced two weeks ago by police in Philadelphia at the Republican National Convention.
Between chants of "Ain't no power like the power of the people," and "Al Gore, Big Oil," many of the brightly clad protesters talked about who they'd be voting for in November.
"At this point, I'm thinking Ralph Nader," said Erysla Mellajoy, 37, of Oakland, Calif. "But if it's down to the wire, I'm going for Gore."
Mellajoy was referring to weeks of opinion polls showing Texas Gov. George W. Bush with a healthy lead over the vice president. She said that, when November rolls around, if Gore is close to Bush in the polls, she would find it very hard to vote for Nader.
The Democrats hope to convince liberal-leaning voters - among them the self-described green activists like Mellajoy - that a President Gore would remain loyal to his environmental message, working to check what is perceived in green circles as rampant global development by large corporations at the expense of human rights.
Mellajoy said she would make her decision late in the game. But where she was on the fence, other marchers seemed to have lost faith in the Democratic party.
Holding a sign reading "Choose your form of oppression," a 29-year-old Oklahoman calling himself Spruce said that, while Nader won't appear on the ballot in his home state, he'll nevertheless find a protest candidate.
"I'm left out in the cold," said Spruce, who added that under no circumstances would he vote for either major party.
Charlie Wilken, 45, of Van Nuys, Calif., was marching inside a cage he built for the protest. It read "Republicrat; Jail for the Poor."
Of the Democrats and Republicans, Wilken said: "There's one or two issues where they may differ. But both parties will bomb Iraq, and both will de-forest the world in the interests of corporate power."
Wilken did allow that there is a little daylight between the Democrats and GOP, saying the parties do differ over the abortion issue.
Standing in front of a phalanx of police in riot gear, Linda Lee said the protests were focusing on her favorite issues: Occidental Oil's plan to drill on U'wa tribal lands in Colombia; political support for the missile defense system, allowing Nader into the presidential debates in the fall, and what she described as a general "anti-corporate ethos."
Lee, 47, of Lampoc, Clif., described herself as a Nader supporter. But she did not rule out a vote for Gore. She'll be assessing the vice president's performance at the convention as well as his views on environmental issues.
Han Shan of the Ruckus Society, the now-famous civil disobedience training group, said the Occidental Oil issue was the dominant protest theme for Monday. U'wa tribesmen in Colombia, Shan said, are threatening mass suicide if Occidental Petroleum drills on their land. Shan said Gore and the Democrats are more likely to listen to the oil company than the native tribesman.
As might be expected, Shan is a solid Nader supporter.
Despite the clash Monday night, the mood among protesters seemed to be one of vague hope - and cynicism - vis-à-vis the Democratic Party. Rather than expressing that cynicism by chaining themselves to downtown storefronts, many activists are leaning toward making themselves heard at a ballot box.