LONDON, Sept. 3 - London's subway network virtually shut down at the height of the rush hour on Monday evening when 2,300 maintenance workers walked off the job in what they said would be a three-day strike over pensions and security.

Transportation officials then closed nine subway lines, the bulk of the system. They said it was too dangerous to keep the network going without the workers, who are responsible for maintaining and repairing tracks, signals, trains and the like. Just three lines - the Jubilee, Northern and Piccadilly lines, which are maintained by workers who belong to another union - were operating Monday night.

Commuters across London left work early in a rush to make it home before 6 p.m., when the strike began. Commuters arriving later found that their stations were locked or - in those stations still operating - that signs had been put up explaining that most of the lines had stopped operating.

Transport for London, the local agency that runs the subway system, predicted that the strike would cause "massive disruptions for millions of Londoners" and urged passengers to seek "alternative routes" - a difficult proposition in a city as large, sprawling and choked with road traffic as London.

The maintenance workers say that if their demands are not met, they will remain off work for three days, and strike again for another three-day stretch next week.

Adding to the general feeling of annoyance, the mayor, Ken Livingstone, said motorists driving into central London during business hours would still have to pay the congestion charge of 8 pounds a day, or more than $16, during the strike.