(Updates with school opening delay, St. Louis county police chief press conference)
By Nick Carey and Carey Gillam
FERGUSON, Mo., Aug 13 (Reuters) - The police officer who fatally shot an unarmed black teenager last weekend in Ferguson, Missouri, was injured in the incident that has sparked racially charged protests, the city's police chief said on Wednesday.
Police Chief Thomas Jackson told a news conference the unidentified officer was treated at a hospital for swelling on the side of his face, one of the few details released about events surrounding the Saturday night shooting of 18-year-old Michael Brown.
After three nights of tense standoffs between protesters and heavily armed police - and instances of violence and looting - Jackson urged demonstrators to rally only during daylight hours. He said it was not a curfew and said that protesters could get their point across with peaceful demonstrations.
The local Ferguson-Florissant school district meanwhile postponed its first day of classes until Monday, cancelling classes for this Thursday and Friday.
In protests on Wednesday in the St. Louis suburb, about 150 demonstrators marched chanting "hands up, don't shoot" and carried signs including "Mike did not have to die."
A handful of police in riot gear were in place at the end of the march. Police from neighboring districts have been brought in to reinforce the Ferguson police department.
Jackson said he could not release any more information on the shooting because witness statements were still coming in.
What police have said is that the shooting followed a struggle between the officer and Brown.
But that, along with much else about the incident, is a matter of dispute. A witness who was walking with Brown at the time has said in media interviews that Brown put his hands in the air and was not struggling with the officer. He said the officer fired multiple times into Brown's head and chest.
The witness, Dorian Johnson, was expected to meet on Wednesday with prosecutors and investigators, local media reported. His lawyer, Freeman Bosley, a former St. Louis mayor, did not immediately answer requests for comment.
Police have declined to release the name of the officer involved in the incident, citing concerns for his safety, a decision that has been criticized by demonstrators who have asked for more transparency. The officer has been placed on administrative leave.
Jackson said his priority was improving race relations in Ferguson, which has seen a stark demographic shift in recent decades, going from mostly white to mostly black. About two-thirds of the town's 21,000-strong population are black. On a police force of 53, three officers are black.
"This is an opportunity to fix what's wrong," he said. He said that the U.S. Department of Justice was advising Ferguson officials on how to improve relations with the community and would work with civil rights groups.
Jackson said investigation would last two weeks.
TRYING TO MAKE SENSE OF IT
Ferguson residents were still trying to make sense out of what happened on Wednesday.
"I cannot condone violence, but I definitely understand where the anger of young black men comes from at times like this. I don't think until you've experienced that kind of treatment that you can understand it," said Mike McCoy, 41, who runs an African-American youth program.
McCoy, who said he has been a victim of racial profiling by local police, was visiting a memorial of candles, teddy bears and messages near where Brown was killed.
National figures from President Barack Obama to civil rights activist Rev. Al Sharpton have called for a peaceful response to the shooting.
About 40 protesters have been arrested since Saturday.
After Tuesday night's protests were broken up by tear gas, a St. Louis County Police officer shot and critically wounded a 19-year-old male who allegedly pointed a gun at the officer. The man, who was identified on Wednesday evening as Esran Britton, had been involved with the earlier protests, police said.
"I have been in fear since Saturday night that something like this would happen," Jon Belmar, the St Louis County police chief, said at an evening news conference. "It's remarkable that we haven't had more problems but I am hopeful the unrest will come to an end soon."
Meanwhile, police in California were investigating a separate incident of an officer fatally shooting an unarmed 25-year-old black man in Los Angeles.
On social media, groups claiming to be associated with the Anonymous hacker activist group called for nationwide protests and threatened to reveal personal information about Ferguson police officers.
The Ferguson police said there have been attempts to hack their website and that it was temporarily disrupted.
Ferguson Police spokesman Tim Zoll said the cyber-threats prompted the decision not to release the officer's name. (Additional reporting by Brendan O'Brien in Milwaukee; Writing by Fiona Ortiz; Editing by Edith Honan and Eric Walsh)