The tube strike has seen London Underground services across the capital hit by a "solidly supported" strike over ticket office closures.
Picket lines outside stations formed last night as members of the Rail, Maritime and Transport union mounted walked out out at 9pm.
The action will last for 48 hours and will be followed by a three-day stoppage next week.
The two sides are embroiled in a fresh row over the closure plans, which the union warned threaten safety as well as almost 1,000 jobs.
LU denied there would be any impact on safety and said ticket office staff would provide a better service if they were moved to other parts of stations.
The RMT said its members were solidly supporting the industrial action, as the union again attacked London Mayor Boris Johnson over the future of ticket offices.
RMT acting general secretary Mick Cash said: "London Underground have dug themselves into an entrenched position and have refused to move one inch from their stance of closing every ticket office, in breach of the agreement reached previously through Acas which enabled us to suspend the previous round of action and in flagrant violation of repeated promises from the London Mayor Boris Johnson that not a single ticket office would be closed on his watch.
"It is scandalous that Transport for London are blowing what we estimate to be hundreds of thousands of pounds on politically-motivated adverts and propaganda designed to deflect attention from Boris Johnson's broken promises.
"RMT could have recommended the suspension of this strike action if LU had responded positively to our proposal to halt the ticket office closures and job cuts, stopping the dire impact they would have the length and breadth of London Underground.
"As a consequence of the management stance, and the broken promises of Boris Johnson, the action has gone ahead and is solidly supported.
"RMT remains available for serious and meaningful talks around our alternative proposals."
Business groups warned that the strikes will cost the capital's economy millions of pounds.
London Underground managing director Mike Brown said: "The RMT leadership appear to remain implacably opposed to the modernisation of the Tube that will radically improve customer service and help us keep fares down.
"For example, at our busiest stations, there will be nearly a third more staff visible and available to provide, on a permanent basis, the face-to-face customer service we offered during the London 2012 Games.
"Visitors to London and people with disabilities will be better looked after than ever before.
"Safety and security will never be compromised. Safety is not controlled from ticket offices but by station supervisors and dedicated control rooms. This will continue.
"Fairness to our staff is also guaranteed. There will be no compulsory redundancies, there is a job for all staff wanting to remain with us and no-one will lose pay as a result of change.
"We have also made significant changes to our original proposals after listening to staff and unions in over 40 meetings, including agreeing that supervisors will not need to 'reapply for their jobs'.
"However, the RMT leadership continues to say 'no' to everything, and they also appear in the context of these changes to be opposed to giving our staff the option of voluntary redundancy.
"Only the RMT leadership know the real motivations behind their actions, but it is infuriating that London's commuters and businesses are the ones who are being forced to pay the price with five days of utterly pointless and disruptive strikes.
"We have asked all the trade unions to continue talking to us this week and we hope that they continue to do so."
The RMT also launched a 48-hour strike from 3am today on the Heathrow Express in a separate row over jobs, pay and cuts.
Large queues built up as early rush-hour passengers waited until 7am for the first Tube trains to run.
At Euston station in north London, customers crowded around the entrance to the Underground, waiting for the clock to tick round.
On the busy Victoria line, where trains normally run every two minutes or so, there was only a 10-minute service when trains finally began running.
At London's Victoria station, passengers pouring off mainline trains were confronted with a wall of people waiting for Tube services.
London Underground said it was running services on nine lines despite the "pointless" strike, although there were no trains on the Waterloo & City or Circle lines.
There were almost 8,000 buses on the roads - the most ever operated in London - after an extra 266 were put into service.
LU managing director Mr Brown said: "Thousands of staff and volunteers are working hard this morning to keep London working and our customers informed in the face of this pointless strike.
"More London Underground staff have come to work this morning than during the strike back in February, and a record number of London buses are operating."
A spokesman for Heathrow Express said: "Despite the strike reducing the company's available workforce from 450 to 150, our trains between Paddington and Heathrow are running scheduled today, thanks to the hard work and commitment of our reservist staff."