Pakistan's martial law administration today placed the country's most prominent opposition leader, Benazir Bhutto, under house arrest, a move that clearly signaled its intention to maintain tight controls on all domestic political activity.

Armed police this morning surrounded the Bhutto family home in the port city of Karachi, and ordered supporters of Bhutto's Pakistan Peoples' Party, who had gathered outside the villa, to disperse. Police and government officials in Karachi and the capital, Islamabad, told reporters that Bhutto would be confined to her home for 90 days.

Bhutto's arrest, only eight days after she returned from exile for the funeral of her brother, demonstrated the military government's determination to prevent any spontaneous political activity while it manages a cautious evolution toward limited civilian rule. In the view of opposition politicians, the arrest also damaged the credibility of those civilian politicians who have cooperated with the military's plan to hand power to a civil administration whose precise powers remain undefined.

In Washington, State Department spokesman Charles Redman expressed "dismay" at Bhutto's arrest, saying it "would appear to be inconsistent" with the Pakistani government's move toward civilian rule.

Some western observers suggested that Bhutto may have, in the government's eyes, violated a tacit agreement in which she was permitted to return to Pakistan but was not to be politically active.

A senior Interior Ministry official, Shah Mahmoud Khurro, had said last week that Bhutto would remain free if she did not "start agitating."

Bhutto had flown to Karachi Tuesday night from her native town of Larkana, where last week's funeral was held. At the villa in Karachi, she addressed hundreds of enthusiastic supporters, denouncing Pakistani President Gen. Mohammed Zia ul-Haq and his martial law government as "repressive" and predicting that popular discontent would grow.

In the same speech, however, Bhutto told her supporters the People's Party would not agitate for an immediate end to martial law but would give the government a chance to return to civilian rule by the end of the year, as promised by Prime Minister Mohammed Khan Junejo.

The Movement for the Restoration of Democracy, a broad umbrella group embracing Bhutto's party and 10 smaller left and center parties, declared in a statement that Bhutto's arrest "exposes the contempt in which Zia holds his own parliament and prime minister." The statement said the arrest had cast doubt on Junejo's assurance of civilian rule within four months and called on him to resign.

A government spokesman, speaking by telephone from Islamabad, said Bhutto had been confined to her home under martial law provisions "to maintain peaceful conditions in the country and preserve national security." But N.D. Khan, a Bhutto spokesman reached by phone in Karachi, said the arrest "shows the government is really shaky, really afraid of Benazir."

Her speech Tuesday night was moderate, and was addressed to some people who had come to her home to pay condolences on her brother's death, Khan said. "It was not inflammatory," he added.

Benazir Bhutto is the daughter of former prime minister Zulfiqar Ali Bhutto, overthrown and eventually executed on murder-conspiracy charges by Zia's martial law government. The enthusiastic welcome she received upon her return last week demonstrated that she retains much of the broad popular support once enjoyed by her father.

Bhutto's return has been widely regarded as an important political challenge to President Zia's plan to return the country to civilian rule, which so far has excluded all established political parties, including Bhutto's.