Prime Minister Zulfiqar Ali Bhutto tonight imposed martial law on Karachi and two other major Pakistani cities in an effort to evade mounting pressure on him to resign.

An Army spokesman said military rule was going into effect immediately in this industrial center and port and in Lahore and Hyderabad, both scenes of violence in the six-week-old political drive against Bhutto.

Some Pakistani and foreign political observers thought that Bhutto was taking a desperate and last-ditch step with the martial-law decree. "The only option left to him, should this fail, will be to resign," one Western diplomat said.

An alliance of 26 labor unions has called a nationwide strike Friday, putting more pressure on Bhutto.

Some observers in Karachi believe that the embattled prime minister will respond by putting the entire country under curfew or martial law. The industrial strike organized by the Pakistan Labor Alliance Karachi yesterday crippled the city.

The alliance's leader, Mohammed Sharif, said he expected "some positive results," meaning Bhutto's resignation within the next 48 hours. But that is not a widely held view.

"It's just not his style," one Western diplomat commented. "He's a fighter and he'll go down fighting."

Bhutto's opponents began demanding his ouster after the March 7 national elections, which they claim he won through massive rigging. They want new elections.

Several informed Pakistanis in Karachi said they believed that even if Bhutto did impose martial law nationally, the country's urban population would defy the army until Bhutto left office.

But the involvement of the rural population, the bulk of Pakistan's 65 million citizens, is not considered likely at this point because of its dependence on large landlords who support Bhutto.

The martial law order had been anticipated in Karachi. The city had been under a total curfew since 6 a.m. today. A curfew was imposed on Hyderabad, an industrial city 100 miles northeast of here on the Indus River, after 12 persons were killed there in demonstrations this afternoon.

(Karachi police reported that crowds defying the curfew stoned vehicles and set street fires, according to United Press International. Hospitals reported seven persons injured in a clash with troops enforcing the curfew. Curfews in the two cities were lifted four hours Thursday afternoon.)

Under the curfew, Karachi was a ghost town. The only people in the streets were aremd troops, police and a handful of civilians with curfew passes. Jeeps and machinegun-mounted trucks rolled through the dusty streets, but there was no shooting.

A few military aircraft used the Karachi airport shuttling passengers among this city, Lahore and the national capital Islamabad.

Bhutto has called a meeting of Parliament members from his Pakistan People's Party in Islamabad Friday. They are expected to express continuing support for him.

A source in Lahore said the strike set for Friday was expected to be highly effective. The Punjab provincial capitol is Pakistan's intellectual center, strongly opposed Bhutto and has been hardest-hit by political agitation.

Labor leader Sharif said, "We don't know what shape this martial law will take," and he added, "We'll have to see if the curfew will be removed now and people will be told to go back to work."

If that happened, he said, he was confident that the majority of the 1.5 million workers on strike in Karachi yesterday would not return to work. "They will not adhere to Mr. Bhutto's rules, martial law or not," he said.

Few observers doubt that Bhutto could impose martial law. Chief of Staff Gen. Zia Haq is a hand-picked supporter, as are several other top officers. The loyalty of junior officers is not as certain.

Realizing this, Bhutto earlier this week approved substantial pay raises for junior officers and enlisted men, intended to narrow the salary gap between senior officers. Importantly, Bhutto has the unquestioned loyalty of the Federal Security Force, a para-military unit of 40,000 men with a reported annual budget of $92 million.

The urban population has been aroused by the prospect of dumping Bhutto and establishing a representative democracy.

"We've had all we can stand of Mr. Bhutto and his dictatorial ways," said a journalist for a semi-offical government paper. "The people of Pakistan have proven they are willing to die for democracy." He cited the mounting death toll, now understood to be over 300, although the offical figure is about half that. "We'll go on dying if we must, until Mr. Bhutto leaves," the newsman added.