Tube services in London were hit today by a "solidly supported" strike over ticket office closures.
Members of the Rail, Maritime and Transport union mounted picket lines outside stations after walking out at 9pm last night.
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.
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."