London: A strike on the London Underground crippled much of the city’s transportation network on Wednesday, disrupting the plans of millions of travelers and forcing commuters to cram into overcrowded buses and trains or walk or cycle to work.

The 48-hour strike, which began Tuesday evening, was called by two unions to protest plans to cut about 950 jobs and close all ticket offices as part of a restructuring that the London transportation authority says could save around 50 million pounds, or about $81 million, a year.

The strike shut down several parts of the system, which normally has three and a half million subway rides each day. The lines that remained open were operating on a reduced schedule.

Despite promises of extra bus service, long, snaking lines built up on Wednesday at stops. Many people faced long walks to work unless they could pounce on one of the city’s shared bicycles.

There was frustration, too, for drivers. “Traffic congestion was far more intense and went on much longer than normal,” said Chris Lambert, a traffic analyst at Inrix, a company that monitors traffic flows. An 11-mile traffic jam on one of the main arteries into the west of the city did not clear until around noon, Mr. Lambert said.

At the Old Bailey, London’s criminal court, a judge commended jurors who appeared despite the disruption, thanking them for showing the “Dunkirk spirit,” a reference to the evacuation of British and some Allied forces from France in 1940 during World War II.

Manuel Cortes, leader of the Transport Salaried Staffs’ Association, one of the unions that called the strike, said there had been overwhelming support for it from his members.

“All we have is a fringe service in the outer suburbs with virtually the whole of central London dependent on a skeleton service,” Mr. Cortes said in a statement. “Over 70 percent of the normal service is at a standstill.”

Transport for London, the city’s transportation authority, described the strike as “completely unnecessary” and said it was running at 35 percent of normal Underground service, with eight of 11 lines and 70 percent of stations operating.

In Parliament, Prime Minister David Cameron condemned the disruption. “There is absolutely no justification for a strike,” he said. “We need a modernized Tube line working for the millions of Londoners who use it every day.”

But politically, Mr. Cameron may gain from the strike if angry travelers blame the trade unions, which have close links to the opposition Labour Party. According to British news reports, Mr. Cameron is considering changes to the law to make it harder to call strikes in some sectors, possibly including transportation.

The government may also welcome the news media focus on the leader of one of the striking unions, Bob Crow, the general secretary of the Rail, Maritime and Transport union, who has a reputation as one of the country’s more outspoken union bosses. He is facing criticism after a newspaper published photographs of him over the weekend on vacation in Brazil as negotiations to avert the strike were continuing.

On Wednesday, Mr. Crow, now back in Britain, staged a verbal confrontation with Mayor Boris Johnson of London, an outspoken critic of the strike, by calling in to a radio show on which Mr. Johnson routinely appears.