Millions of commuters are facing delays and frustration as a strike by London Underground station staff shuts down much of the city's subway network.

Several central London subway lines were closed during rush hour Monday morning, forcing many to walk, use overcrowded buses, or work from home if possible. Other Underground lines offered only sporadic service.

Transport for London said service would be greatly reduced until Monday evening. The strike also closed subway links from terminals 4 and 5 at London Heathrow airport and connections at London's major train stations.

Transport unions are protesting job cuts and ticket-office closures, citing safety issues.

London's subway system handles more than 4 million journeys during a typical business day.