(CNN) -- Occupy activists tossed pipes, bottles, burning flares and other objects Saturday at Oakland police, who responded by using tear gas and smoke grenades and arresting more than 100 demonstrators, city and police officials said.
Oakland has been a flash point of the Occupy movement since October when police used tear gas to break up demonstrators who refused to leave downtown. One demonstrator, a Marine veteran of the war in Iraq, suffered a skull fracture after being hit with a police projectile, according to a veteran's group. Police said they acted after the crowd threw paint and other objects at officers.
On Saturday, police made mass arrests following an afternoon clash with protesters near the Kaiser Convention Center and then later outside a downtown YMCA, according to a police statement.
The tension began Saturday around noon when about 250 activists gathered in Frank Ogawa Plaza. They were joined later by another 200 people as they marched around the city.
Police said the protesters "began tearing down perimeter fences around" the Kaiser center around 2:30 p.m., with officers ordering the protesters to leave the area about 20 minutes later.
"The City of Oakland welcomes peaceful forms of assembly and freedom of speech, but acts of violence, property destruction and overnight lodging will not be tolerated," the statement said.
On its Twitter feed, Oakland police wrote that people near the YMCA were being arrested for unlawful assembly. Police claimed, too, that parts of the Oakland Museum and Kaiser Center were "severely impacted," with people cutting and tearing fences.
Occupy protesters also blocked traffic at major intersections and attacked a KTVU television truck, damaging the truck and breaking windows, the station said. No injuries were reported.
By nightfall, Occupy protesters moved toward Oakland City Hall, according to the group's Twitter feed.
"Amazing day. We didnt get in the building, but fought like our future depended on it," the group tweeted.
Posts on Occupy Oakland's Twitter feed claim that police met the protesters "with munitions and violence." One read: "#OccupyOakland being teargassed smoked bombed & shot at w rubber bullets."
Oakland police said they used only smoke and tear gas. They did so after warning activists who had begun "destroying construction equipment and fencing" around the Kaiser center, authorities said. Officers were "pelted" with bottles, metal pipes, rocks and burning flares, according to another police statement released earlier in the day.
While there was no immediate word on injuries to protesters, Oakland police said that three of their officers were hurt in the earlier clash.
After that incident, roughly 500 activists returned to Ogawa Plaza, according to a police statement. Aerial video footage showed them later on the move and a post on Occupy Oakland's Twitter feed indicated they were planning to march to a "second building" -- an apparent reference to the YMCA.
While touting the action as "Move-In Day" on their website, occupyoaklandmoveinday.org, organizers did not state what "large, vacant" building they planned to occupy. They also acknowledged that "like the encampment at Oscar Grant Plaza, the building move-in is not legal."
But the group said the move was necessary, in part because "since November, the city of Oakland and its police force have made it impossible for us to meet, to serve food, and to provide a place for people to stay."
"Occupy Oakland is about people providing for themselves and for others, since it is clear that the system can no longer provide for them," the group said on its website. "It is a place where people who are fed up can come together and develop new forms of struggle."
In a letter to Mayor Jean Quan, the city council and the Oakland police department, protesters threatened several actions if authorities try to "evict us again." They included "blockading the airport indefinitely, occupying City Hall indefinitely, shutting down the Oakland ports" and getting help from the activist hacking group known as Anonymous.
In mid-December, Occupy protesters shut down the Oakland port terminal.
Following October's clashes with protesters, Police Chief Howard Jordan said "all allegations of misconduct and excessive uses of forces are being thoroughly investigated by internal and external investigative sources," according to a letter posted on the department's website.
The website also solicits "input ... about police interactions with protesters during the Occupy Oakland protests." People are asked to contact the independent investigator who is looking into authorities' actions in October and November 2011.
CNN's Greg Morrison contributed to this report.