Millions of commuters in London endured travel chaos on Tuesday as a 72-hour strike by maintenance workers closed most of the city's underground rail network.

As the strike entered its second day, Transport for London (TfL) said the disruption was "severe and unacceptable", with trains suspended on all but three of the 12 lines.

Many of the three million passengers who use the network each day were forced to find alternative routes to work and the city's buses were packed.

"We share Londoners' view that this disruption is intolerable, as it serves no purpose," a TfL spokesman said.

Prime Minister Gordon Brown called on the workers to end the strike.

"This is a wholly unjustifiable strike," he said at his monthly news conference. "It is causing an enormous amount of trouble to the people of London and disruption to the business of this city.

"They should get back to work as quickly as possible."

About 2,300 staff at collapsed contractor Metronet walked out at 6 p.m. on Monday in a row over jobs and pensions.

Even though the strike is due to end at 6 p.m. on Thursday, the chaos is expected to drag on into Friday.

The RMT rail union said it would hold talks with TfL, Metronet and its administrator later on Tuesday to try to end the walkout.

"We hope that Metronet and its administrator will now take our members' legitimate concerns seriously," RMT General Secretary Bob Crow said in a statement.

The RMT says it wants guarantees that the collapse of Metronet will not lead to job losses or pension cuts.

It went under in July after banks denied it access to funds amid a projected overspend of 2 billion pounds.

Metronet is responsible under a 30-year public-private partnership contract for the infrastructure of nine Tube lines.

TfL and London Mayor Ken Livingstone say they have given the union guarantees over jobs.

A second 72-hour strike is due to start on Sept. 10.