LONDON (AP) _ The heart of London became a battleground Saturday, and fashionable shopping and theater districts packed with locals and tourists suddenly found themselves at the mercy of roving gangs.

It was a scene more reminiscent of street fighting in Beirut or Israel's occupied territories than London's favorite tourist haunts: Trafalgar Square, Regent Street, Piccadilly Circus and Leicester Square.

Running battles between rock-throwing militants and riot police left a trail of injury and destruction, from overturned and burned-out cars to smashed windows and looted shops.

The militants, who broke off from a protest by more than 40,000 people demonstrating against a new nationwide local tax, smashed plate-glass windows at some of the world's most famous stores, including Liberty's, Aquascutum, Wedgwood and Mappin and Webb.

''The mindless mob, although a small minority, has taken over,'' said a uniformed policeman at the intersection of Oxford and Regent streets.

On Charing Cross Road, a mile away, looters carried away expensive musical instruments even as police with riot shields tried to clear the area. Two china shops on Regent Street also had their display windows pillaged.

Many of the broken windows set off burglar alarms, which mixed with the constant wail of police sirens and fire engines.

The rioting broke out in late afternoon as the stores prepared to close for the weekend. It spread into the evening and overtook theatergoers headed into London's West End for a Saturday night out.

Several theaters kept their doors shut at the height of the rampage, leaving ticket-holders standing outside. Some of them got caught between the protesters and the police.

Most shows went on, although some started late.

One woman dressed in a silk dress and fur jacket walked up to a riot policeman holding a shield outside the Palace Theatre at the corner of Shaftesbury Avenue and Charing Cross Road. ''Excuse me,'' she said, ''have you got any idea how long this is going to last?''

At the Duke of York Theatre in St. Martin's Lane, 650 people were locked in as rioting flared outside during a performance of the play ''Shirley Valentine.''

''It was like being under siege,'' said General Manager Andy Phipps.

One target of the rioters was the Renault showroom on Charing Cross Road near the Leicester Square subway stop, one of five in central London that were sealed off by police. The showroom's floor-to-ceiling windows were smashed, and so were the windows of every car inside.

Rocks, cans, bricks and bottles were thrown through the windows of banks, restaurants, shops, the royal jewelers Garrards, Tower Records and nightclubs, including the popular Stringfellows. Garbage cans heaved by the protesters ended up amidst spring fashions and floral displays in many showrooms along Regent Street.

Charing Cross Road and St. Martin's Lane, which runs from Leicester Square to Trafalgar Square, were littered with rubble, broken glass and torched cars.