Eight police officers were hurt and 18 protesters arrested in Seattle last night when a May Day rally that began peacefully turned violent after dark, with crowds hurling rocks and bottles at police who responded with pepper spray grenades.

Protesters threw rocks, bottles and chunks of asphalt at officers, officials said, smashing store and car windows, overturning trash cans and lining up newspaper display racks to block streets.

Police in riot gear, some riding in armored SWAT vehicles, responded by repeatedly firing "blast ball" grenades, which emit smoke tinged with pepper spray.

"We're a bigger, better city than this," Mayor Mike McGinn said at a news conference. "I'm disappointed that this is the picture the world sees of us."

Most of the 150-200 protesters who had stayed on the street after darkness fell for a "non-permitted" protest had dispersed by midnight.

"We did not start to take action until that group itself started to act violently towards the officers and the community at large," Assistant Police Chief Paul McDonagh said.

The day started with a peaceful march in support of immigrant rights, part of May Day rallies in cities across the U.S. West planned by a coalition of organized labor activists, students, civil rights advocates and members of the clergy.

In Los Angeles, thousands of protesters marched through downtown waving American flags and carrying signs with the slogan, "Stop deportations."

One police officer said that unofficial estimates put the size of the crowd in Los Angeles at roughly 3,500 people. No arrests were reported.

In Arizona, where a state crackdown against illegal immigration was signed into law three years ago, several hundred people joined a late-afternoon rally outside the state Capitol in Phoenix, ahead of a march through downtown.

McGinn said Seattle officials had not expected violence but had wanted police to maintain a visible presence for the rally following the Boston Marathon bombing two weeks ago.

"The Boston situation is really on all of our minds," McGinn said. "I thought it was important to ... be in a position to move promptly if there were law breaking."

Of the eight officers who were hurt most suffered only scrapes and bruises, police officials said, although a policewoman was hit in the knee by a large chunk of asphalt.

A bipartisan group of U.S. senators in April introduced an 844-page bill, backed by President Barack Obama, that would rewrite many of America's immigration laws.

A centerpiece of the measure would create a path to legal status and ultimately citizenship for many of the 11 million undocumented immigrants in the United States.

It also aims to secure the U.S. border with Mexico against illegal entry and to make it easier for industry, particularly high-tech businesses and agriculture, to hire workers from abroad when needed.

Reuters