Hundreds of police moved in, dividing the crowd, and officers apparently fired several rubber bullets. There were some arrests.
It was the first real sign of trouble in a day that remained calm and orderly despite the fact that demonstrators were constantly shouting at, swearing at and taunting police officers.
"Oink, oink, bang, bang, every day the same old thing," protesters chanted, as they headed past rows of closed shops in downtown Los Angeles during the afternoon.
Through the day, they were met everywhere with the very heavy police presence that has marked the Los Angeles Police Department's approach to controlling the demonstrations - officers in riot gear, most gripping truncheons, some rifles, and a police helicopter whirring overhead.
But despite the emotional overflow from a confrontation Monday night in which some rally- and concert-goers threw things at police, who retaliated with rubber bullets, pepper spray, and baton-wielding officers on horseback, the rallies yesterday had produced few incidents.
In fact, the early march to the Rampart station was a nearly textbook example of peaceful demonstration and police restraint.
The Rampart march grew as it moved into that largely poor, predominantly immigrant downtown neighborhood - reaching an estimated 600 people by midday. Thirty-seven people who sat in front of the steps and refused to move were arrested without trouble, bringing the total convention-demonstration arrests to about 200.
Although protesters chanted "No justice, no peace, no murdering police," on the way to the station, once there, they were calm.
A Rampart police officer, Michael Moore, spoke continuously to protest organizers, and helped to facilitate their march back to the park where they started after all the seated demonstrators had been arrested.
"Let me know when you're ready to move," he told Lisa Fithian, one of the lead protest organizers. "We'd like the entire group to leave as a group so you can leave in solidarity and safety."
The protesters left calmly - after holding hands across four sides of a busy intersection and singing the song that is their anthem here:
We have come too far
We won't turn around
We'll flood the streets with justice
We are freedom-bound.
At the same time, protesters were gathering at Pershing Square, about a half-dozen blocks from the convention center, for a rally on ending police brutality, corruption, use of excessive force, and the death penalty.
Some participants held signs emphasizing the LAPD's actions this week, such as the yellow one that read, "Does this look like Berlin 1930's?" or the scrawled initials LAPD - with words spelled out beneath each letter, "License to Attack Peaceful Demonstrators."
One red sign screamed, "Racist, Sexist, Anti-Gay, LAPD Stay Away!"
As dozens of police lined up on the lawn of Parker Center in several rows, some wearing bulletproof vests, protesters en masse made an obscene hand gesture and booed.
"LAPD you can't hide. We charge you with genocide," they chanted. "We're fed up, won't take it no more."
The crowd was mostly young - with about 50 masked people dressed in black wedged into the center, nearly hidden by large anarchist-themed banners.
"Whoever they vote for, we are ungovernable," read the large capital letters on one long black banner.
Behind the banners, one young man burned a flag in front of Parker Center. Police did not react.
Nita Lelyveld's e-mail address is email@example.com