The pre-dawn hanging execution of deposed Prime Minister Zulfiqar Ali Bhutto left Pakistan stunned today. But sporadic demonstrations here and in the neighboring Indian state of Kashmir raised the specter of a sharper reaction Thursday when Bhutto supporters heve called for a national day of prayer.

Nearly 500 Bhutto supporters defied martial law regulations today to shout antigovernment slogans and battle police in Liaqat Park here, the place where Bhutto began the movement that brought down Gen. Ayub Khan's government 10 years ago.

"Death to Zia and Zia's children," women shouted, referring to Pakistan's military ruler, Gen. Zia ul-Haq, before police moved in swinging their metal-tipped bamboo staves.

Abdul Hafeez Pirzada, Bhutto's former finance minister who acted as his defense attorney, was carried shoulder high by the weeping, emotional crowd, mostly women.

As they waved the flag of Bhutto's Pakistan People's Party, the crowd shouted "Revenge, revenge, revenge." Some women tore at their clothes and taunted the police, "Shame on you, you couldn't save hime."

The reaction in the capital city of Islamabad, 15 miles north of here, was more subdued but still angry. Employes in government offices wept openly, and an elderly woman snapped, "This is the most disgraceful day in Pakistan's history."

One man warned, "Now this country will be like Iran."

The most violent demonstrations, though, were in the heavily Moslem Indian state of Kashmir, which borders Pakistan.Three persons were killed and seven injured in clashes between pro-Bhutto demonstrators and police in scattered parts of the state.

Bhutto, a charismatic, Western-educated leader who ruled Pakistan from 1971 to 1977, was hanged under unusual secrecy at 2 this morning in the central jail here after being convicted a year ago of ordering the murder of a political opponent.

As he stood on the scaffold and the hangman strapped his legs tightly together, Bhutto cried out, "Oh Lord, help me, for I am innocent."

Bhutto looked gaunt from his 18 months in prison. Although prison authorities allowed him a last bath to wash away the filth of his last 11 days in a dirty and bug-infested cell, they refused to bring him a final meal supplied by friends.

The hanging was carried out despite pleas for clemency from world leaders including President Carter and Pope John Paul II.

The body of Bhutto, who was 51, was quickly buried in the family graveyard near the town of Larkana.

From this walled cemetery, Bhutto may pose a greater threat in death than he ever did in life to Zia, who has called for national elections in November. Zia ousted Bhutto in a coup in July 1977, following an election in which Bhutto is widely charged with having rigged the vote.

Bhutto's party has called prayer meetings for Thursday afternoon all over Pakistan. Although the military government has ordered that news of these meetings be censored from newspapers here, large turnouts could signal massive dissatisfaction with Zia's Islamic government.

Foreign diplomats in Islamabad expressed fears that the swift, secret execution would lead to great future unrest.

"If there had been a lot of advance warning, there would have been a law and order problem. But the way they have done it, Bhutto is going to grow into a legend which will one day backfire on them," one foreign observer was quoted as saying.

Most interesting of all will be the reaction of the armed forces. There were known to be deep divisions over hanging Bhutto - both on the question of whether he was really guilty of conspiring to murder a political opponent and over the wisdom of killing him even if he was. Now Gen. Zia has cast the die, and the military must face the consequences.

Mumtez Ali Bhutto, a leading lieutenant in the People's Party and the dead prime minister's cousin, commented: "Bhutto is dead, but the problem for them has been magnified a million-fold. Let us see how they deal with it now."

The military government was determined to keep the hanging secret, and they abandoned prison regulations to do it. Only Bhutto himself, and the two family members who were under house arrest - his second wife, the Begum Nusrat, and his daughter Benazir, 25 - knew that the execution would take place at 2 a.m. instead of the usual hour of dawn. Security around Bhutto's wife and daughter was strengthened so no word could leak out of the execution.

Bhutto was writing out his will when, just after midnight, armed troops took up positon around the jail. Two Pakistani journalists who were seen watching the activity were arrested and held in a nearby jail. They were released at dawn.

Bhutto signed his will in the presence of the magistrate who would shortly watch him die. Then, with his arms tied behind his back, he was taken in slow procession to the scaffold. Witnesses said his six escorts moved with their eyes downcast.

Bhutto's body, the head hooded, dangled from the noose for the required 30 minutes before the hangman cut him down, and he was laid out in a simple coffin. An Army truck waited in the yard to carry it to the nearby Chaklala Air Force Base, adjoining Islamabad International Airport. Just before 4 a.m., a military aircraft took off for the the town of Sukkur in the province of Sind, and another truck drove the coffin to Bhutto's birthplace.

There, according to the official statement, the body was handed over to Bhutto's first "traditional" wife, the Amir Begum, who wears the veils of purdah. Two uncles were also informed.

The Amir Begum looked into the coffin. "I saw the face. They had shaved him and it was obvious that he had been hanged. But he looked as innocent as a flower," she said.

Bhutto's second wife and his daughter were not permitted to attend the funeral. They spent the harrowing death watch under house arrest. CAPTION: Picture 1, Supporters of the late Zulfiqar Ali Bhutto mourn his death in Liaqat Gardens in Rawalpindi following Bhutto's hanging at the city's main jail. AP; Picture 2, ZULFIQAR ALI BHUTTO ... a quick burial; Picture 3, GEN. MOHAMMED ZIA UL-HAQ ... shows no mercy; Picture 4, Bhutto's two sons tell reporter of their grief over the execution; UPI; Picture 5, right, Bhutto's wife, the Begum Nusrat Bhutto, cries after learning her husband was hanged. AP; Map 2 X marks Bhutto family village where former prime minister was buried, By Dave Cook-The Washington Post; Picture 6, Pakistani police charge a crowd in Rawalpindi that is causing a traffic jam while offering prayers for Bhutto. UPI