Los Angeles - San Francisco filed a first-of-its-kind lawsuit on Tuesday against President Donald Trump over his decision to withhold federal funds from sanctuary cities."The president's executive order is not only unconstitutional, it's un-American," said city attorney Dennis Herrera in announcing the suit filed in federal court. "That is why we must stand up and oppose it."We are a nation of immigrants, and a land of laws. We must be the 'guardians of our democracy' that President Obama urged us all to be in his farewell address."The lawsuit, the first such challenge to Trump's executive order issued last week, urges an injunction against the president and his administration to prevent them withholding federal funds from American cities that protect undocumented immigrants.These so-called sanctuary cities refuse to assist or co-operate with federal officials in enforcing immigration laws.There are some 300 such cities across the United States and many have vowed to stand up to Trump's order aimed at cracking down on illegal immigration.Trump's administration argues that these sanctuary cities harbor criminals but advocates say they offer needed protection to undocumented residents who contribute to the community.Herrera said San Francisco, which has about 30,000 undocumented residents, receives more than $1.2bn a year in federal funding, most of which goes to health care and other programs."This lawsuit is not a step I take lightly," he said. "But it is one that is necessary to defend the people of this city, this state and this country from the wild overreach of a president whose words and actions have thus far shown little respect for our Constitution or the rule of law."