By Dietrich Knauth and Jack Queen (Reuters) -A landmark trial kicks off on Monday over the use by Donald Trump's administration of National Guard forces to support its deportation efforts and quell protests in Los Angeles, in a legal challenge to the U.S. president's break from long-standing norms against deploying troops on American streets. The three-day non-jury trial before San Francisco-based U.S. District Judge Charles Breyer will determine if the government violated a 19th century law that bars the military from civil law enforcement when Trump deployed the troops in June, as the state of California claims in its legal challenge. Los Angeles experienced days of unrest and protests sparked by mass immigration raids at places where people gather to find work, like Home Depot stores, a garment factory and a warehouse. The administration denies that troops were used in civil law enforcement and plans to show that they were protecting federal property and U.S. Immigration and Customs Enforcement (ICE) agents. Many of the troops have been withdrawn, but California's Attorney General Rob Bonta said on Monday that 300 National Guard members are still going on immigration raids and restricting civilian movements in the state. "The federal government deployed military troops to the streets of Los Angeles for the purposes of political theater and public intimidation," Bonta said in a statement. "This dangerous move has no precedent in American history." California and Gavin Newsom, the state's Democratic governor, have asked Breyer to prohibit the troops from directly participating in domestic law enforcement activities. California and Newsom say the National Guard is accompanying ICE agents on raids and assisting in arrests, in violation of the Posse Comitatus Act of 1878 and other laws that forbid the U.S. military from taking part in civilian law enforcement. The Republican president ordered 700 Marines and 4,000 National Guard troops to Los Angeles in June against Newsom's wishes. California's lawsuit ultimately seeks a ruling that would return its National Guard troops to state control and a declaration that Trump's action was illegal. The trial before Breyer will have limited impact, however, on Trump's plan to deploy hundreds of National Guard troops to Washington as part of a crackdown on violent crime in the U.S. capital city despite statistics showing that violent crime there hit a 30-year low in 2024. "I'm deploying the National Guard to help reestablish law, order and public safety in Washington, D.C.," Trump said on Monday. "And they're going to be allowed to do their job properly." Unlike in California and other states, where the governor typically decides when to activate Guard troops, the president directly controls the National Guard in Washington. A ruling against the administration could restrict the actions of those troops and constrain Trump if he deploys such forces to police other American cities. Trump said his efforts to fight crime will go beyond Washington and he mentioned Chicago, Los Angeles, New York and Baltimore. Trump's decision to send troops into Los Angeles prompted a national debate about the use of the military on U.S. soil and inflamed political tensions in the second-most-populous U.S. city. A U.S. appeals court has allowed Trump to retain control of California's National Guard during the legal challenge. (Reporting by Dietrich Knauth and Jack Queen in New York and by Jan Wolfe in DC; Editing by Tom Hals, Tom Hogue, Will Dunham and Andrea Ricci)