Tube strike causing misery for Londoners
There's also a bus strike in west and south-west London
A strike by London Underground workers is disrupting travel across the capital.
Services on the vast majority of Tube lines are suspended, with a very limited operation elsewhere, according to Transport for London's (TfL) website.
This has led to severe traffic jams in some parts of the capital.
Rail, Maritime and Transport (RMT) union members are taking industrial action over issues including jobs and pensions.
There is also disruption to bus services in west and south-west London and parts of Surrey due to a strike on Friday and Saturday by drivers who are members of the Unite union.
Sixty-three bus routes are being affected.
Mainline train services started later than normal on Friday due to the knock-on effect of Thursday's RMT strike at Network Rail and train operators across the country.
Just 70% of services will run during Friday as a whole.
A further walkout on Saturday will reduce service levels to 20%.
RMT general secretary Mick Lynch claimed London Underground workers' pensions are at risk.
Speaking at a picket line in Acton, west London, he said: "We're out on strike today because we haven't got an agreement from the TfL LUL (London Underground Limited) about the future of the pension scheme, which is under threat and, at the moment, there's negotiations going on between the Government, the Treasury, and Mayor (Sadiq) Khan's office about the future funding of London Underground.
"the union has to show it is "deadly serious" about the livelihoods of its members"
"They've cut £2 billion from the funding and that has put at risk our members' pensions.
"They want to decimate the pensions and change it into an entirely different, cheaper scheme, which will make all our members poorer in retirement and probably pay more while they are working.
"We haven't gotten an agreement. There's also threats to terms and conditions. We wanted a promise that terms and conditions won't be diluted and there is a threat of a loss of jobs in certain sections of the Underground."
He said he is "very sorry" that people are being inconvenienced by the strikes but defended the action, adding that the union has to show it is "deadly serious" about the livelihoods of its members.
The Transport Secretary Grant Shapps has told union bosses to "get out of the way" and allow members to vote on newly proposed pay deals.
London Mayor Sadiq Khan said: "The only reason for the strikes in recent weeks in London is because of the conditions the Government is trying to attach to a funding deal, and the trade unions are concerned about the consequences of those conditions on their members.
"This is about pension concerns that trade unions have.
"I don't want these conditions imposed on our transport workers."
There are also strikes on a number of national rail lines tomorrow.