Iran's new military government arrested the country's leading political opposition figure yesterday shortly before he was to have announced a new hard-line program aimed at overthrowing Shah Mohammad Reza Pahlavi.

Twenty minutes before he was scheduled to hold a news conference calling for "subordination" of the shah to the will of the Iranian people, National Front leader Karim Sanjabi was taken into custody at his home by army officers and agents of SAVAK, the secret police.

Members of his family said late last night they had received no word of his whereabouts. "We still don't know where he is. We're waiting, just waiting," said a daughter.

Sanjabi, who returned Friday from Paris after meeting with exiled Shiite Moslem leader Ayatollah Ruhollah Khomeini, was held on unspecified charges. He was driven away in an unmarked automobile just as reporters began arriving for the news conference in the northern Tehran suburn of Manzerieh.

The principal spokesman for the National Front, Dariush Foruhar, who is a member of the opposition group's seven-member executive council, was taken into custody with Sanjabi.

Sanjabi's followers said his arrest increased the possibility of further violent demonstrations today during a worker's strike called by political and religious opposition leaders.

Although the government gave no reason for the arrests, observers said they apparently reflect a decision by the new military government to limit opposition to what the government considers acceptable limits of protest. The government had shown restraint in dealing with National Front leaders when they had not yet swung over to Khomeini's hard-line demands for the overthrow of the shah.

Some of Sanjabi supporters also speculated that the arrests may reflect a determination of the miltary government to test the strength of the National Front, whose still unmeasured constituency has been riding the coattails of the more powerful religicus opposition.

Members of the National Front who were waiting in Sanjabis's spacious home, which is not far from the shah's palace, said an army officer they identified only as Maj. Gen. Rahimi and four SAVAK agents walked in the front door and, saying they wanted to see Sanjabi, headed to a downstairs back room where Sanjabi and Foruhar were conferring. An aide of Sanjabi's said Rahimi is attached to the shah's palace guard.

[United Press International identified him as Rahimi Larijani, Tehran chief of the secret police.]

A half-dozen uniformed soldiers waited outside while Sanjabi and Foruhar were led outside by the authorities. Sanjabi, trailed by reporters, was led to the waiting car and driven away. He did not speak and the authorities who led him away refused to comment.

Dozens of other reporters were restrained by SAVAK agents at the end of the quiet residential street until well after the two National Front leaders had been removed.

Manuchehr Keyhani, a physician and a spokesman for the National Front, said the agents asked Sanjabi what he intended to say at the news conference, and that Sanjabi replied that he planned to answer reporters' questions and reiterate a three-point declaration agreed upon in Paris between himself and Khomeini.

Before the Paris conference, Sanjabi and other National Front leaders had advocated negotiating with the shah for a compromise political solution to the growing rebellion. They suggested a provisional government could be established and free elections held that presumably would lead to a constitutional monarchy.

However, Khomeini held firm to removal of the shah and creation of an Islamic republic. The resulting communique adopted that stand, as well as calling for a referendum on the future of the monarchy.

Members of the National Front who were in the room said that Rahimi then read the statement and told Sanjabi and Faruhar that they were under arrest.

Left on a coffee table in the living room were copies of a statement that National Front officials said Sanjabi had intended to read, as well as copies of the three-point declaration.

Sanjabi's statement specifically ruled out any National Front participation in a government under the shah's monarchy. This apparently dashed any prospects of an attempt by the shah to defuse the present crisis by resorting to a coalition government including his longstanding but relatively moderate political opponents.

"The national and Islamic movement of Iran would not agree to any kind of government with the presence of the illegal monarchical regime," said the statment.

Other points of the National Front's new platform condemned the monarchy as illegitimate, both "legally and religiously" because of its "constant violations of constitutional law, suppression, corruption, and surrender to foreign powers." The final point called for a referendum on forming an "Islamic democracy," a code word for replacing the monarchy with an ill-defined republic under the influence of the country's traditional religious leaders.

In another statement, the National Front said it was obvious that the current troubles are "a basic and constant crisis" resulting from the shah's authoritarian rule. The statement termed the recent riot "a revolutionary movement emerging from the depths of our cultural, religion and national identity."

It said the shah himself had stated he could not ignore this revolutionary message, but that "the shah in recent speeches has shown he has not heard the message of our revolution by appointing army leaders as a military government and continuing to massacre the people."

A statement by Iranian dissident Ahmad Bani-Ahmad, who recently resigned his parliamentary seat in protest against the government, blamed the news military prime minister, Gen. Gholam Reza Azhari, for the deaths of protestors in continuing disturbances.

The official Iranian radio said rioting broke out in Isfahan yesterday, without mentioning casualties. It also said two people died in the Persian Gulf port of Bandar Shahpur in anti-government violence.

Bani-Ahmad accused the military government of deliberately cutting off international telecommunications links of foreign news organizations in Iran and holding more than 2,000 political prisoners.

Earlier in the day, the shah received four new Cabinet ministers, parliamentary representatives and Islamic ambassadors in a ceremony marking a Moslem feast day celebrating the high point of the Hajj pilgrimmage to Mecca.

Informed observers said the shah appeared a different man. Signs of mounting strain were obvious as he greeted his visitors with a grim, tightlipped expressions on his face.He rarely smiled and spoke in a soft, weak voice with none of the animation he usually shows at such ceremonies.

As he listened to speeches in his honor, the shah's attention seemed to wander. After bowing low and kissing the shah's hand in a reception line, senior Iranian generals watched their leader with obvious concern.

The shah, wearing a relatively simple army uniform with no ostentatious decorations, appeared especially grim as he listened to a speech by the dean of the Moslem diplomatic corps wishing him and his son, Crown Prince Reza, "long life and prosperity."