London Underground lines were suspended on Monday as workers went on strike, causing travel disruption for commuters.
Members of the Rail, Maritime and Transport (RMT) union including drivers, signallers and maintenance workers launched a series of strikes over pay and conditions.
The action started on Sunday but the biggest impact will be between Monday and Friday.
Transport for London (TfL) warned there will be few or no services between Monday and Thursday.
Picket lines were mounted outside Tube stations on Monday.
There will also be no Docklands Light Railway services on Tuesday and Thursday because of a strike by RMT members in a separate pay dispute.
TfL has offered a 3.4% pay rise which it described as “fair” and said it cannot afford to meet the RMT’s demand for a cut in the working week.
Nick Dent, London Underground’s director of customer operations, said union demands for a cut in the 35-hour week were “simply unaffordable” and would cost hundreds of millions of pounds.
The last Tube-wide strike was three years ago, over pay and pensions, but Mr Dent said this week’s action will be different because separate groups of workers will walk out on different days.
“It will be very damaging for us,” he added.
An RMT spokesperson said: “We are not going on strike to disrupt small businesses or the public.
“This strike is going ahead because of the intransigent approach of TfL management and their refusal to even consider a small reduction in the working week in order to help reduce fatigue and the ill-health effects of long-term shift work on our members.
“We believe a shorter working week is fair and affordable, particularly when you consider TfL has a surplus of £166 million last year and a £10 billion annual operating budget.”
