* Iraq war veteran hit in the head by tear gas canister
* Tensions building in Atlanta, Orlando and Baltimore (Updates Oakland protests, changes bylines, previous ATLANTA)
OAKLAND, Calif, Oct 26 (Reuters) - Over 1,000 activists protesting economic inequality and corporate greed massed on Wednesday night in a downtown Oakland plaza from which they had been evicted a day earlier in clashes with police that left 85 people arrested and one critically hurt.
The severe injury of Scott Olsen, 24, a former U.S. Marine who friends said served two tours of duty in Iraq, became a rallying cry among Occupy Wall Street supporters in Oakland and elsewhere as organizers took to Twitter and other social media urging protesters back into the streets.
In Portland, Oregon, a crowd estimated to number at least 1,000 joined in a march organized by the AFL-CIO labor federation in support of the anti-Wall Street movement. And Twitter buzz suggested the turnout may have gotten a boost from outrage generated by news of the injured Oakland veteran.
Supporters in New York voted on Wednesday to send $20,000 and 100 tents to their peers in Oakland, according to a Twitter message from a protester identified as J.A. Myerson and re-tweeted by the Occupy Wall Street group.
The liberal activist group MoveOn.org said it was creating a "rapid response ad" from video footage of the "brutal crackdown" Tuesday night in which Olsen was hurt.
Rally organizers said Olsen was struck in the head by a tear gas canister fired by police at protesters trying to move back into a downtown plaza where a makeshift encampment had been forcibly removed earlier.
A spokesman for Highland General Hospital in Oakland confirmed Olsen was in critical condition from injuries sustained in the protest but could not say how he was hurt.
Acting Oakland Police Chief Howard Jordan told a news conference his department was investigating the injury to Olsen as a "level one" incident, the highest priority for an internal police inquiry.
He declined to confirm whether Olsen was struck with a projectile fired by law enforcement but said Oakland police did launch tear gas and so-called "bean bag" munitions on Tuesday when demonstrators defied orders to disperse.
Jordan, acknowledging his department has received complaints of excessive force during the protests, said his officers were under orders to accommodate peaceful rallies and marches. But he added that "no camping will be allowed overnight" on public property.
The altercations erupted on Tuesday when about 1,000 activists sought to retake an outdoor plaza adjacent to City Hall that police had cleared earlier that day. Police arrested 85 people.
On Wednesday night, a crowd of at least 1,000 demonstrators were allowed back into the square. Some immediately ripped down a fence erected to close off a grassy area after authorities had removed tents and sprayed disinfectant chemicals.
But the crowd was otherwise peaceful and activists met to discuss strategy, including a proposal to call for a general citywide "strike" next week. Except for a helicopter hovering overhead, police remained absent from view.
BADLY HURT AFTER TWO TOURS IN IRAQ
Keith Shannon, who said he served with Olsen in Iraq, told Reuters his friend suffered a 2-inch skull fracture and brain swelling and had been sedated in the hospital's emergency room trauma center while neurosurgeons decided whether to operate.
The hospital declined to comment on those medical details.
"The irony is not lost on anyone here that this is someone who survived two tours in Iraq and is now seriously injured by the Oakland police force," said his friend, Adele Carpenter, who spoke to Reuters by phone from the hospital waiting room.
The "Occupy Wall Street" protests, which began in New York City on Sept. 17, take issue with a financial system they say benefits corporations and the wealthy. They are critical of U.S. government bailouts of big banks, high unemployment and economic inequality.
Loosely organized protest groups have since sprung up across the United States and in countries around the world. Tensions were building in several cities where authorities have been treading a fine line between allowing peaceful protest and addressing concerns about trespassing, noise and safety.
In an early morning raid in Atlanta, police evicted dozens of protesters from a downtown park and arrested 53 who refused to leave. They were allowed to camp in the park for three weeks but Mayor Kasim Reed said he decided to evict the protesters because of fire code violations and crowd control issues.
In Orlando, demonstrators had been complying with orders to vacate a park overnight and left their belongings, only to have police confiscate the property. And in Baltimore, the city ordered protesters to drastically reduce the number of people who camped overnight from roughly 200 to two people in a single tent. Protesters were given a Wednesday deadline to comply.
In the birthplace of the demonstrations, New York City, authorities have largely averted confrontation. Over 700 protesters loudly but peacefully marched through lower Manhattan on Wednesday to denounce for-profit healthcare.
The New York demonstrators are camped in a privately owned park in the city's financial district, and Mayor Michael Bloomberg has said there was little the city could do until the park owners, Brookfield Office Properties, file a complaint. (Additional reporting by Mary Slosson in Los Angeles, Barbara Liston in Orlando, David Beasley in Atlanta, Dan Cook in Portland, Jason Tomassini in Baltimore and Chris Francescani in New York; Writing by Steve Gorman and Barbara Goldberg; Editing by Greg McCune and Jackie Frank)