Prime Minister Margaret Thatcher, visiting the riot-wracked Toxeth district of Liverpool, emphasized the need to protect police and restore law and order in Britian's cities today as her government authorized police use of military-style riot equipment if necessary in response to widespread urban disorders.

William Whitelaw, who is in charge of law enforcement as home secretary, said tonight that police may use water cannons and plastic bullets against rioters if chief constables decide they need them.

Whitelaw told a meeting of Conservative members of Parliament that the government had also offered armored vehicles to the police and Army camps to prison authorities for detention of rioters if the country's already overcrowded jails cannot hold the rioters.

Disturbances broke out again in several industrial cities in northern England late tonight -- the 11th consecutive night of urban violence. But police officials said the disorders were much less widespread and not nearly as serious as previous nights, with London particularly quiet.

For the third night in a row, the most serious trouble was in Leicester, where hundreds of black and white youths battled the pllice with bricks and bottles. Marauding youths smashed store windows in Nottingham and Liverpool.

Police in Leicester and Leeds had faced attacks early this morning, including the throwing of molotov cocktails and the looting and burning of many stores. During the past two nights there also have been firebomb attacks on police stations in Burmingham, Wolverhampton and Derby, but they caused no injuries or serious damage.

"There is no future for anyone unless law and order is upheld," Thatcher declared after touring Toxeth, which suffered the worst of the violence that eventually spread to more than 30 cities and towns, including 20 neighborhoods in London.

Hundreds of police have been injured in the upheaval, the worst in mainland Britian in recent history, while thousands of people have been arrested and tens of millions of dollars in property damage has been caused.

Thatcher viewed the riot damage and talked to some Toxeth residents through the window of her bullet-proof black Jaguar during a breakfast-hour motorcade through the port city. After a long morning meeting with city officials and community leaders at Liverpool city hall, Thatcher was raucously jeered by a large crowd protesting her economic policies that have creat ed an industrial slowdown. A tomato and toilet paper rolls were thrown at her car.

Some community leaders said afterward they did not think Thatcher had understood concerns they expressed about police-community relations, high youth unemployment and housing problems. Thatcher told reporters at a press conference that "the first thing to do is to protect the police," who are doing a difficult job.

She acknowledged that the past 10 days had been "the most worrying" since she became prime minister, but said, "Law and order will be upheld. People are learning to cope with the situation, and I hope peace will soon be restored."

Informed sources said Queen Elizabeth II, who has studied government reports on the rioting and watched the violence on television, has become more concerned about it than any problem the country has faced in years. Thatcher will be discussing it with her Tuesday night at the prime minister's weekly meeting with the queen.

On Thursday, Thatcher faces a major political test in a day-long parliamentary debate on the riots. Some right-wing members of Parliament from her own Conservative Party, complaining that the government has not gotten tough enough with the rioters, have advocated creation of special squads of riot police or putting the Army on alert. Opposition Labor Party leaders have charged that Thatcher's economic policies are responsible for much of the high unemployement they blame as one cause of the upheaval.

Thatcher is reported by sources to have agreed in weekend consultations with Whitelaw that the initial outbreak of rioting that may have sprung from racial tensions, social problems and conflict with the police has become, on more recent nights, wanton lawlessness.

This also is the judgment of many police officials. Chief Constable Charles McLaughlin told reporters after weekend window smashing and looting by 200 black and white youths in Nottingham that "those who are doing this give us trouble 365 days a year."

Police officials contacted late tonight in several jurisdictions, including Scotland Yard in metropolitan London, said they were not yet planning to use plastic bullets, water cannons or the other new equipment offered by the government. They said they are waiting to see whether the disturbances continue to ebb or flare again. Government officials also said the Army camps are not yet needed as detention centers.

But the opposition Labor Party's law enforcement spokesman, Roy Hattersley, said tonight that what Whitelaw was offering chief constables could represent a "radical altering" of policing in Britian that he feared could produce aggressive riot squads like those elsewhere in Europe.

Whitelaw said riot trained police in some jurisdictions would be used to reinforce overextended officers in neighboring areas and a "national reporting center" at Scotland Yard would coordinate riot control information for the country.

Plastic bullets have previously been used by the police and Army only in Britich-ruled Northern Ireland. Ulster police have refused to use a type of heavy-stream water cannon that Whitelaw said would be among those tested for possible use against rioters. During the disturbances in mainland British cities, police have been armed only with truncheons and riot shields.

Sources said Thatcher has pushed for decisions on some of the new law enforcement measures for announcement in time for Thursday's parliamentary debate.She and some other members of her Cabinet believe this needed to be done quickly for police and public morale, according to these sources, while expensive measures to cope with unemployment and inner-city deterioration can come later, if there is money to pay for them.

"Unemployment is always distressing and it is very much easier to talk about it than finding a solution to it," Thatcher said today in Liverpool."Jobs cannot be pulled out of hats."