WASHINGTON/NEW YORK - Thousands marched in Washington, New York and Boston on Saturday to protest killings of unarmed black men by police officers.Organizers said the marches were among the largest in the recent wave of protests against the killings of black males by officers in Ferguson, Missouri; New York; Cleveland; and elsewhere. The protests were peaceful, although police in Boston said they arrested 23 people who tried to block a highway.Decisions by grand juries to return no indictments against the officers involved in the deaths of Michael Brown in Missouri and Eric Garner in New York have put police treatment of minorities back on the national agenda."We're going to keep the light on Mike Brown ... on all of the victims. The only way you make roaches run is to keep the light on," said civil rights leader the Rev. Al Sharpton, whose National Action Network organized the Washington rally.Sharpton urged Congress to pass legislation that would allow federal prosecutors to take over cases involving police violence.