London Underground said it was running half of Tube services despite a strike by thousands of workers in a row over ticket office closures.
The company said two thirds of Tube stations were open, a record number of buses were running and almost 90% of the usual number of Oyster cards were used on the network.
Services were running on nine of the 11 lines and more Tubes were in operation than during the last strike earlier this year, said LU.
The Rail , Maritime and Transport union said its members were "solidly" supporting the 48-hour strike, which started at 9pm last night.
The two sides are embroiled in a deadlocked 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's assertion that its members were solidly supporting the industrial action was denied by London Mayor Boris Johnson.
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."
London Underground m anaging 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."
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.
Shadow transport secretary Mary Creagh said: "It is wrong millions of Londoners are facing travel misery today. Strikes are always a sign of failure and both sides must get round the table urgently and sort this out as quickly as possible."
Mr Johnson said: "The idea that this is a solidly-supported strike is farcical. This action is the result of a minority of just one union, the RMT, who are refusing to see the logic of what we are trying to achieve.
"We have got more staff out on the network than we did before, more services running and two thirds of stations open, and I am very grateful to London Underground workers who have come out to keep the capital moving.
"I apologise for the disruption that this has caused people, but I'm very impressed by the fighting spirit Londoners have shown in getting to work today. I thank them for their patience.
"We can't, as some would have us do, just stand by and force Londoners to pay for ticket offices. A resource which currently costs Londoners money, and where less than 3% of journeys begin, can be delivered better and far more efficiently with these changes.
"Modernising the network as we are seeking to do will save £50 million a year, money that will be reinvested in the things our customers do actually want whilst helping to keep fares down.
"There are no compulsory redundancies, no-one who wants to play a part in the future of the Tube will lose their job as a result of our plans.
"I urge the RMT leadership to do the right thing - get back around the table; not the wrong thing - muscle flexing in the race for the RMT leadership. Start thinking about hard-working Londoners and small businesses across the capital and call off this pointless strike."
Prime Minister David Cameron tweeted: "It's unacceptable that millions of people are having their lives disrupted by today's Tube strike in London."
The RMT also launched a 48-hour strike from 3am today on the Heathrow Express in a separate row over jobs, pay and cuts.
Mr Cameron's official spokesman told a regular Westminster media briefing: "Many Londoners today will have had to put up with very significant disruption to their journeys in and out of the capital.
"I think the Prime Minister thinks that it is wrong that they are facing disruption this week as well as next.
"I think the Mayor of London is absolutely right to note the way London is determined to keep going about its business."