The US government shut down at midnight after Congress failed to resolve a partisan standoff over immigration and spending.
In a late-night vote, Senate Democrats blocked a bill that would have kept the government running for another four weeks.
A flurry of last-minute negotiations failed to beat the deadline.
Democrats have tried to use the Friday night funding deadline to win concessions from Republicans, including an extension of an Obama-era program protecting some young immigrants from deportation.
The program is set to expire in March. Republicans sought more time for talks, but Democrats refused.
The shutdown is only the fourth government closure in 25 years. It will only partially curb government operations. Uniformed service members, health inspectors, and law enforcement officers are set to work without pay.