Disruption is expected on the Tube network until Tuesday evening because of the walkouts
The first in a series of Tube strikes begins later, with services expected to be disrupted until Tuesday night.
Up to 200 Jubilee and Northern Line maintenance staff will walk out for 24 hours from 1900 BST in a pay row.
Thousands of their colleagues across the network will stage a separate 24-hour strike at 1700 BST on Monday, with drivers, signallers and station staff walking out at 2100 BST.
They are unhappy about plans to scrap 800 jobs in Underground ticket offices.
The Rail Maritime and Transport (RMT) and the Transport Salaried Staffs Association (TSSA) unions are fighting the prospect of having fewer employees at stations, saying security could be compromised for passengers.
But London Underground has insisted all stations will still be staffed and has pledged there will be no compulsory redundancies.
An extra 100 buses will be put into service on Monday and Tuesday, and there will be room for 10,000 more journeys on riverboat services on the Thames.
Certain taxi ranks will be marshalled and escorted bike rides will be provided for commuters using alternative methods of transport.
Mayor Boris Johnson said: “Londoners are a hardy bunch and I am sure a Tube strike will not deter us from getting around.
“I have asked Transport for London to pull out all the stops, but we must be clear that the RMT and TSSA plan to inconvenience Londoners for no good reason.”
But RMT General Secretary Bob Crow said: “Instead of meaningless PR gimmicks from the mayor he should start telling his officials to take this dispute seriously and he should also start putting Tube safety before the dash to slash budgets.
“He should get a grip on the lethal consequences that his officials’ Tube cuts will have for both staff and the travelling public.”
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