LONDON--Prime Minister Margaret Thatcher, claiming "I am the true disarmer," said yesterday she will make sure Britain deploys new U.S. nuclear cruise missiles this year despite growing public opposition.

In a live television interview one week after her triumphant tour of the Falkland Islands. Thatcher said the missiles are vital to counter a Soviet build-up.

"These are not extra missiles," Thatcher declared in the hour-long interview with the Independent Television Network's "Weekend World" program. "As they go in, one for one, older ones will be taken down.

"So they're not increasing the number of nuclear weapons at all," she said. "They're substituting a modern weapon for an older one. We needn't deploy any if the Soviets can be persuaded to negotiate and take their SS-20s down."

Thatcher faces increasing criticism, from the opposition Labor Party and the strong grass-roots Campaign for Nuclear Disarmament movement as Britain approaches a deadline for deploying new nuclear weapons to counter the estimated 620 Soviet SS-20 missiles aimed at Western Europe.

Britain is one of the Western European members of the North Atlantic Treaty Organization that have agreed to deploy 572 cruise and Pershing 2 missiles by the end of 1983 unless Soviet and U.S. arms negotiators in Geneva reach a missile reduction agreement.

"I don't understand the unilaleralists," Thatcher said. "I am the true disarmer. I keep peace and freedom and justice," she added.

"One-sided weakness by the West makes war more likely," Thatcher said. "An effective nuclear deterrent...has been so powerful because these weapons are so awful it has kept the peace for 27 years.

"You have to deter a potential aggressor," she said. "Weakness would tempt him. Strength stops him."

Thatcher, whose term ends in May 1984, denied widespread rumors she will call early elections to capitalize on popularity she accrued from her trip last week to the Falklands.