LONDON — Campaigning for re-election, Mayor Livingstone of London called Conservative opponent Boris Johnson a “joke.” Today, Mr. Johnson is the one who should be laughing.

After parlaying appearances on a television quiz show into a political career, Mr. Johnson won one of Europe’s biggest electoral prizes by outwitting opponents who dismissed him as a buffoon.

Mr. Johnson beat the Labour Party’s Livingstone by focusing on crime — even as police reported that offenses overall were down. He benefited from the increasing unpopularity of Prime Minister Brown and his ruling Labour Party, especially in London’s outer suburbs.

“Boris plays the buffoon, but in fact he’s a very shrewd and sharp guy,” a senior research fellow at University College London and a former colleague when they were journalists, Brian Walker, said in an interview. “He can be ruthless.”

During his campaign, Mr. Johnson promised to make streets safer for London’s 7.5 million residents by hiring more police and providing them with mobile metal detectors. He pointed to the deaths of 27 youths last year in gun and knife attacks, the most in at least five years, to argue that London had become less safe. He also emphasized a police probe of City Hall into whether community groups misused or stole grant money.

Mr. Johnson promised to “sweep out the current mayor’s Marxist cabal” and to end waste and “sleaze” in City Hall, a reference to the police investigation.

Mr. Johnson, who takes over today, won 48.4% of the vote, beating Mr. Livingstone, who has held the mayor’s post since it was created in 2000, with 42.6%. In local elections in England and Wales also on May 1, Labour took 24% of the vote compared with the Conservatives 44%, the worst performance for at least four decades, BBC projections show.