MANCHESTER, England (AP) _ Prime Minister Margaret Thatcher, seeking re-election to a third term, told a rally of 200 supporters Friday that a vote for her was a vote against socialism.

''Vote Conservative. Anyone who does not vote that way may risk letting in socialism by inadvertence or default,'' she told Conservative Party supporters while speaking outside a social club in the Manchester suburb of Hazel Grove.

''That is the message for everyone. If they happen to say at the same time, 'We love Maggie,' that will do for me,'' she said through a megaphone.

Mrs. Thatcher ignored about 40 demonstrators carrying banners of the main socialist opposition Labor Party and the extreme left Socialist Workers Party.

The Conservative Party has argued that votes for the centrist Liberal- Social Democratic Party alliance will risk splitting the right-of-center vote and let in a Labor government. The Tories won Hazel Grove in the 1983 election with a thin 2,000-vote majority over the alliance.

Mrs. Thatcher took to the road as opinion polls showed the Labor Party cutting into the Conservatives' strong lead. Over the last week, the Tories have had a 10 percent to 14 percent lead in most polls.

A Marplan poll in Friday's Guardian newspaper gave Conservatives 41 percent of the vote, down 2 points; Labor 33 percent, up 3 points; and the alliance 21 percent, down 4 points. The poll of 1,517 voters, taken Thursday, had a 3 percent margin of error.

A Harris poll for TV-AM completed over five days this week and released Friday put the Conservatives further ahead: Conservatives 43 percent, Labor 36 percent and the alliance 20 percent. The number of people questioned and the poll's margin of error were not released.

Mrs. Thatcher was traveling Friday aboard a bulletproof bus emblazoned with the slogan ''Forward With Maggie In The Northwest.''

She swept through the outskirts of Manchester, the big industrial city of northwest England, visiting offices, factories and a farm.

It was her first full day on the road amid one of the tightest security operations ever seen in a British election.

Mrs. Thatcher is a declared target of the outlawed Irish Republican Army, and in 1984, a year after winning a second term, she narrowly escaped death in an IRA hotel bombing in the southern city of Brighton.

She plans, as do the other party leaders, to campaign outside London five days a week until the June 11 election.

The demonstration Friday was a surprise because Mrs. Thatcher's daily itinerary is a closely guarded secret. Journalists traveling in two buses behind her blue bus are not told of the itinerary until the night before.

The demonstrators at Hazel Grove said they got word late Thursday evening from a local Labor Party member that Mrs. Thatcher was coming.

''There's always a grapevine,'' said Denis Butcher, a member of the Hazel Grove Conservative chapter who themselves were only told of the campaign visit Thursday afternoon.

As the mainly elderly Conservative Club members thronged around Mrs. Thatcher, the demonstrators chanted ''Maggie 3/8 Maggie 3/8 Out 3/8 Out 3/8''

She pressed on into the crowd of admirers, getting separated at times from her husband Denis, 72, and some of the security men.

During her speech, the demonstrators stood about 25 yards away and chanted repeatedly, ''Tory scum 3/8 Tory scum 3/8''

Labor leader Neil Kinnock took his campaign Friday to the Welsh valleys where he was born and popped into his local pub for a pint of beer and a meat pie. Stepping inside with him were 26 photographers and three TV camera crews.

With Britain's defense a key campaign issue, the alliance stood by its proposal to cancel the U.S.-made Trident nuclear system, ordered by the government to replace Britain's aging Polaris ballistic system.

Mrs. Thatcher on Friday attacked Labor's commitment to unilateral nuclear disarmament and the alliance's plans to scrap Trident, saying Britain's national security ''is now only safe in Conservative hands.''