Oakland police Chief Howard Jordan said at least four people were arrested in a confrontation between police and protesters near 14th Street and Broadway shortly after noon today.
Jordan said the demonstrators were arrested for offenses ranging from vandalism to resisting arrest.
He said the confrontation began when protesters started throwing objects at officers who were trying to make arrests and disperse the crowd.
Officers then "deployed a small amount of gas" to disperse the crowd, Jordan said. He did not specify whether it was tear gas.
Speaking to reporters at a briefing shortly after 3 p.m., Jordan said, "We're making more arrests now as we speak," but didn't provide further details on the location or nature of the new arrests.
Jordan said there had been "minor acts of vandalism and graffiti" in downtown Oakland throughout the day, including at a Bank of America branch at the Kaiser Center on Lakeshore Drive and a Bank of the West branch at 2127 Broadway.
Jordan said the "tempo" of the protesters seems to be "more assertive and aggressive" than at prior demonstrations, so police have responded by having a more visible presence to try to prevent problems.
He said Oakland police called in mutual aid at 9 a.m. because they observed that there were "multiple, simultaneous events" that were stretching the department's resources thin.
The agencies providing the aid include the Hayward, Fremont, Union City and Newark police departments, the California Highway Patrol and the Alameda County Sheriff's Office.
Jordan declined to disclose how many officers are involved in monitoring the protests. He said he is not aware of any injuries to officers.
Hundreds of protesters began marching through downtown this morning in preparation for the noon May Day rally in the area of 14th Street and Broadway and Frank Ogawa Plaza.
Shortly before 11:30 a.m., about 30 protesters were dancing to music in the middle of the intersection.
About an hour later, dozens of officers in riot gear were at the intersection, and several loud bangs were heard.
Demonstrators had smashed the windshield and slashed a tire on a van belonging to local TV station CBS5 that was parked at 14th and Broadway.
Police spokeswoman Johnna Watson said protesters had surrounded the van while a reporter was inside. The reporter was not hurt in the confrontation, she said.
"That's unacceptable," Watson said of the incident. "The Oakland Police Department won't tolerate violence against other protesters or news media or police."
Later this afternoon, police sent out an email asking media not to park near 14th Street and Broadway.
Oakland police have been preparing for today's protests, and said some planned marches and rallies have permits but others do not.
"We will not tolerate destruction or violence," city officials said in a news release this morning.
Many of the protests in Oakland, including a march slated to start at 3 p.m. at the Fruitvale BART station, are expected to converge at Frank Ogawa Plaza around 6 p.m.
Jordan said police expect up to 1,000 people to show up for the 6 p.m. gathering. He said he anticipates intermittent traffic disruptions related to the protests throughout the evening.
Oakland-based rapper Boots Riley spoke at the noon rally, saying the purpose of today's protests is to rally "for the workers and for the people who are fighting for their rights."
He described the demonstrations as part of a "militant radical labor movement to change society so we get the profits we helped create."
A 30-year-old Oakland man who declined to give his name but said he is an unemployed airplane mechanic said he came to the rally "to protest economic injustice and corporate greed and the police state we're existing in now."
Another Oakland resident, 32-year-old Jesse Smith, described himself as "a conservative Republican" but said he supports Occupy Oakland, in part because it provided important social services to residents at its former encampment in the plaza.
Smith said he is "against the rhetoric" of the Occupy Wall Street movement nationwide but supports specific actions by Occupy Oakland that he thinks are making the city better.