A protester walks away after placing a smoke device on the ground in front of police officers during a May Day march that began as an anti-capitalism protest and turned into demonstrators clashing with police lies on the ground next to police batons, Wednesday, May 1, 2013, in downtown Seattle. AP Photo/Ted S. Warren A protester who was arrested during a May Day march that began as an anti-capitalism protest and turned into demonstrators clashing with police lies on the ground next to police batons, Wednesday, May 1, 2013, in downtown Seattle. Police face off against demonstrators during a May Day protest in Seattle Wednesday, May 1, 2013. Police used "flash bangs" and pepper spray against some protesters who pelted them with rocks and bottles late Wednesday, as violence erupted during May Day in Seattle. Several dozen protesters, many using bandanas to cover their faces, began clashing with police in downtown Seattle hours after a peaceful immigrant-rights march ended. AP Photo/Jordan Stead, seattlepi.com Car wash workers with the Community Labor Environmental Action Network (CLEAN) Carwash Campaign march during a May Day rally in downtown Los Angeles on Wednesday, May 1, 2013. Thousands joined May Day rallies in dozens of cities from Concord, N.H., to Bozeman, Mont. The generally lively gatherings across the country were marred late Wednesday as several dozen protesters - some covering their faces with bandanas - began pelting police in Seattle with rocks and bottles and officers responded with pepper spray and "flash bang" grenades - releasing smoke, a flashing light, and a loud noise. Eighteen people were arrested. The spurt of violence came hours after Seattle's peaceful march for immigration reform by thousands of protesters had ended and was reminiscent of the gathering a year ago when some protesters broke windows and set fires. We're a bigger and better city than this. I look at this and I am disappointed that this is the picture the world sees of us," Seattle Mayor Mike McGinn said. And in New York, thousands of demonstrators marched in downtown Manhattan waving banners and banging on drums in a scene reminiscent of Occupy Wall Street's heyday. The size of the May Day crowds paled in comparison to the massive demonstrations of 2006 and 2007, during the last serious attempt to introduce major changes to the U.S. immigration system. Now, immigrant advocacy groups are focusing heavily on contacting members of Congress, using social media and other technology to target specific lawmakers. Gabriel Villalobos, a Spanish-language talk radio host in Phoenix, said many of his callers believe it is the wrong time for marches, fearful that that any unrest could sour public opinion. Demonstrators waved American flags and signs with messages such as "Stop deportations." In downtown Chicago, U.S. Sen. Dick Durbin told thousands of demonstrators that America had a "once-in-a-lifetime opportunity" to change immigration laws. "We need to seize that opportunity," said the Illinois Democrat, who is part of a bipartisan group of eight senators who introduced the legislation last month. Associated Press writers contributing to this report included Meghan Barr in New York, Morgan True in Concord, N.H., Lauren Gambino in Salem, Ore., Gene Johnson in Seattle, Sara Burnett in Chicago, Edwin Tamara in Los Angeles and Alexandra Tilsley in Denver.