In open defiance of martial law and warnings by President Mohammed Zia ul-Haq that protesters will be dealt with harshly, leaders of Pakistan's banned political parties began a national campaign of civil disobedience today and held illegal rallies calling for an end to military rule.

Rejecting a promise made by Zia on Friday that he will hold elections and disband martial law by March 1985, the opposition demanded immediate restoration of the 1973 constitution and a return to a parliamentary form of government with free elections.

More than 10,000 supporters of the eight-party Movement for the Restoration of Democracy coalition crowded around the tomb here of the nation's founder, Mohammed Ali Jinnah, on Pakistan's 36th independence day. They chanted slogans calling for an end to martial law as scores of riot policemen watched from the fringes.

Several opposition leaders who had evaded police raids on their homes last night appeared at the rally to condemn Zia's military rule and then slipped by the police cordon to return to hiding.

They included Ghulam Mustafa Jatoi, leader of the banned Pakistan People's Party, who called on the demonstrators to raise their right arms to Allah in a vow to save Pakistan and its constitution. The crowd roared its approval and chanted "Down with martial law" and "Long live the MRD."

The outpouring was striking for a country governed by strict martial law, and all the more so since Zia had warned in his televised speech Friday that he would not tolerate civil unrest between now and the time at which he proposes to introduce his power-sharing reforms in 18 months.

Zia promised to hold national elections and return Pakistan to civilian rule within three months after seizing power in a military coup on July 5, 1977. He has made similar promises several times since. He now says he will amend the constitution to share power with a prime minister, but also strengthen the presidency with the power to dismiss the prime minister and dissolve the largely consultative National Assembly.

Demonstrations were also held in Lahore and in Sukker, 300 miles north of here in Punjab Province. Police there used tear gas and riot batons to disperse a crowd that had been listening to a speech by Hazar Khan Bijaraini, another People's Party leader and governor of Sind Province under prime minister Zulfiqar Ali Bhutto, whom Zia ousted in 1977. Bhutto later was hanged for alleged political excesses.

Arrested in Karachi for violating martial-law prohibitions against demonstrations were two members of the Movement for the Restoration of Democracy, Secretary General Khwaja Khair-Uddin and Abed Zubedri. In Sukker, 20 people were arrested during the protest.

Police arrested 200 persons Sunday during clashes with antigovernment demonstrators across Pakistan, and opposition sources said at least 45 persons were injured, United Press International reported.

Hundreds of people stoned a police van in Rawalpindi, adjoining the capital of Islamabad, when retired Gen. Tikka Khan, former chief of the Army and a leader of the banned Pakistan People's Party, was arrested.

Pakistan People's Party Secretary General Farooq Leghari was arrested in Lahore, the provincial capital of Punjab, along with 16 other people.

A large number of people were arrested in Peshawar, near Afghanistan, and in Qetta and several other cities of Baluchistan Province on the Afghan border, but no precise figures were available.

Authories also blocked demonstrations in Hyderabad, 100 miles north of Karachi, and in Badin, 75 miles east of Karachi, UPI said.

Although the demonstration at Jinnah's majestic white marble tomb was marred by several clashes between movement activists and stick-wielding supporters of Zia's regime, the police took no action except to explode one tear gas canister to break up an unruly crowd at the main gate.

After briefly being scattered by the pro-Zia demonstrators under a flurry of blows from hardwood sticks, the opponents of martial law regrouped and surrounded Jatoi and other opposition leaders as they carried a wreath into the tomb. Charging Zia with "betraying the country," Jatoi declared, "We sacrificed for this country. We have come here for democracy, not for martial law." He called for a boycott of the local elections that Zia promised to hold by the end of this year.

Other speakers demanded the hanging of "those who betrayed Pakistan in the name of martial law," and called for the release of Bhutto's daughter, Benazir, who has been under house arrest for six years.

Encouraged by the demonstration, which he afterward termed "the start of a new movement in Pakistan," Jatoi announced that another illegal rally will be held Monday in central Karachi.

The movement has also called on Pakistanis to refuse to pay taxes until martial law is lifted, and to conduct grass-roots protests throughout the country.

Some opposition activists privately voiced their concern that the movement cannot sustain its protest in the face of widespread apathy. They said an awareness is growing in Pakistan that Zia may impose such strict qualifications on candidates that opposition contenders would be barred from participating in elections.

The opposition leaders said they will try to build on that resentment before elections are held.