Millions of commuters in London endured travel chaos on Tuesday as a 72-hour strike by maintenance workers closed most of the 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 3 million passengers who use the network each day were forced to find alternative routes to work, with many packing on to buses.

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

About 2,300 staff at collapsed contractor Metronet walked out at 6 p.m. (1 p.m. EDT) 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.

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

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

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

TfL and Mayor of London Ken Livingstone say they have already given such guarantees.

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

Metronet is owned by WS Atkins, Balfour Beatty, Bombardier Transportation (the rail equipment unit of Bombardier Inc.), EDF Energy (part of EDF SA) and Thames Water.