David Cameron described travel disruption caused by the Underground strike in the capital as "unacceptable" on Twitter.
It's unacceptable that millions of people are having their lives disrupted by today's Tube strike in London.
Thousands of staff and volunteers are working hard to keep the capital working during the "pointless" Tube strike, London Underground said.
TfL said the Victoria line was now running fully with the Central line running to Holborn and the Jubilee line running through from Wembley Park to Stratford.
London Underground managing director Mike 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 Transport for London spokesman said the RMT could not "claim a shut down in any sense of the word."
Passengers formed long queues in London Bridge underground station on the first morning of the Tube strike.
Commuters have been told to expect severe disruption, with TfL advising passengers to plan ahead.
A 48-hour Tube strike has created "total chaos" for Londoners just trying to get to work, one commuter has said.
Emily Toner told ITV London that she was forced to hire a Boris bike because of the crowds at Liverpool Street station:
When arriving at Liverpool Street at the Tube, I discovered the gates were still not open so I resorted to using a Boris bike to get into work.
Total chaos has been caused to good people just trying to get to work.
Commuters have reported having to wait a long time for black cabs at Paddington station this morning.
Staff at the station warned passengers to expect lengthy queues yesterday.
Long queues have formed at bus stations as commuters avoid the London Underground.
TfL said a record 7,961 buses were being used today.
Commuters faced long queues at Harrow-on-the-Hill station on the Metropolitan line today as the first morning of a 48-hour Tube strike got underway.
Transport for London said its ambassadors will help keep Tube stations open and passengers on the move during the 48-hour walkout by London Underground workers.
TfL said a record 7,961 buses were running in London after 266 were added to the service.
Tube strike: Licensed & trained TfL Ambassadors will help keep Tube stations open & provide travel info & advice to customers.