London’s Night Tube began with 50,000 people expected to use the service each weekend.
Mayor Sadiq Khan was on board the first Victoria line service in the early hours of Saturday morning.
Around 100 British Transport Police (BTP) officers are patrolling the network, which operates through the night on Fridays and Saturdays.
Superintendent Chris Horton, the officer responsible for policing the Underground, insisted there was “no reason” why passengers would not be as safe as during the day.
He said the force would focus on “being visible” and ensuring it was “able to intervene in places that are likely to see significant issues”.
All-night services also started on parts of the Central line, where previously some passengers had to get their last train before midnight.
The Jubilee, Northern and Piccadilly lines will follow in the autumn.
London Underground (LU) expects 50,000 people to use the Night Tube each weekend, rising to 200,000 once all five lines are open.
There are six trains per hour through central London between 12.30am and 5.30am.
Mr Khan chatted to passengers using the first train, which departed from Brixton, south London at 12.34am with passengers ranging from boisterous revellers to calm groups and individuals.
“You’re making history,” he told one couple.
Speaking as the train made its way towards north London, Mr Khan told the Press Association: “You can feel the buzz, you can feel the vibe. People are really excited.
“What’s important is we got the detail and the planning right.
“I’m really pleased that 100 days or so after becoming the mayor we’ve got that right.”
Mr Khan’s predecessor, Boris Johnson, announced in September 2014 that the service would begin on September 12 2015, but a bitter dispute with the rail unions delayed the project.
The driver of the first train said the service was “great for London” and insisted he would be able to adjust to working through the night.
Asked if he was concerned about feeling tired, Daniel George, 28, from Cheshunt, Hertfordshire, said: “No, you change your sleeping pattern to work around that sort of thing.”
He continued: “It’s great for people not having to worry about the last train. “It’s a shame I have to be driving the train and not taking advantage of it but I know it’s great for London.”
A recent study by business membership organisation London First estimated the Night Tube could be worth £77 million each year to the capital’s economy by 2029.