Washington  - US President Barack Obama moved Friday to quell a racial firestorm over his remarks about the stupidity of police, phoning both an angry black Harvard professor and the white officer who arrested him.

Obama stopped short of an apology for his remarks this week, as had been demanded by Massachusetts police unions earlier Friday.

But in a surprise appearance before the White House press, the first black US president conceded that he had expressed himself poorly when he declared that Cambridge police had "acted stupidly" in arresting Professor Henry Gates, a leading authority on African- American history and personal friend of Obama.

"I want to make clear that in my choice of words, I think, I unfortunately, I think, gave an impression I was maligning the Cambridge Police Department and (the arresting officer) specifically."

"I could have calibrated those words differently," Obama said, admitting he had contributed to escalating the fury over the incident.

He promised that the three men - the president, Gates and Sergeant James Crowley - would meet at the White House to defuse the tension, perhaps over a beer.

For the first time since taking office, Obama is embroiled in a controversy over racism, despite the popular notion that his election had moved the United States into a post-racial era.

Last year, the presidential candidates ran campaigns remarkably free of racial invective, despite Obama's historic candidacy.

There was controversy over perceived anti-white and anti-American remarks by his Chicago pastor, the Reverend Jeremiah Wright.

But at election day in November, Obama proved he was a bridging, unifying force, attracting white voters in less diverse Midwestern states as well as black voters in the deep South.

At issue in the current issue was the July 16 arrest of Gates and the role played by Crowley, an experienced officer who by all accounts has led efforts within the Cambridge police force to train colleagues to refrain from racial profiling.

The grey-haired Gates, 58, is widely known for his narrations of black history specials on television and his academic achievements at Harvard.

Crowley was responding to a call from a neighbour who reported that two black men were trying to break into Gates' Cambridge home. In fact, it was Gates himself, arriving home late at night from China, where he had been filming a public-television documentary.

His front door was stuck shut, and Gates, who walks with a cane, enlisted his taxi driver to help pry it open. When police arrived, they demanded that Gates identify himself, and a verbal confrontation ensued.

Crowley arrested Gates for disorderly conduct and took him to the police station to book him. The charges were later dropped amid the outcry over the arrest of such a prestigious black university professor.

On Wednesday night, Obama jumped into the fray. While admitting he didn't know all the facts of the case and that he was biased because of his friendship with Gates, the president said: "The Cambridge police acted stupidly in arresting somebody when there was already proof that they were in their own home.

"What I think we know, separate and apart from this incident, is that there is a long history in this country of African-Americans and Latinos being stopped by law enforcement disproportionately. That's just a fact."

Obama stood by his guns on Friday, insisting that as president he had a duty to weigh in on such racial incidents and use them as "teachable" moments.

"Even when you've got a police officer who has a fine track record on racial sensitivity, interactions between police officers and the African-American community can sometimes be fraught with misunderstanding," Obama told reporters. "Race is still a troubling aspect of this society, whether I were black or white."

He called for the country to "spend a little more time listening to each other" and less on "flinging accusations."

Obama said his conversation with Crowley convinced him that Gates and Crowley - "two good people" - both shared fault in the incident.

"I continue to believe ... that there was an overreaction in pulling Professor Gates out of his home to the station. I also continue to believe ... that Professor Gates probably overreacted as well," Obama said.

Crowley insists that Gates provoked his own arrest by not cooperating and becoming verbally belligerent, and dozens of police union leaders backed him up Friday, insisting that race had not played a role in the arrest.

Gates has demanded an apology.

"If this can happen to me in in Harvard Square, this can happen to anybody in the United States," he said. "What it made me realize is how vulnerable black men are ... to capricious forces like rogue policemen." (dpa)