The death toll in antigovernment demonstrations in Iran rose again yesterday, and government officials in Washington expressed concern that if the demostrators achieve their goal - the overthrow of Shah Mohammad Reza Pahlavi - the ultimate result will be a takeover by a pro-Soviet government.

Major demonstrations were reported in a number of Iranian cities and towns, with the most violent apparently occurring in the western town of Paveh, 23 miles from the Iraqi border.

News agencies quoted a government spokesman as saying 11 demonstrators were killed, but press reports reaching Tehran put the figure at 30 to 40. Sources differed on the identity of the killers. Some said a band of several hundred pro-shah Javanrood tribesmen attacked Paveh no horseback. Others said security police fired on the demonstrators.

Shiite Moslems, who object to the modernization program being pushed by the shah, are leading the oppositionto his government, and it is this that is causing the most concern in Washington.

Iran, with its vast oil wealth, its 1,500-mile border with the Soviet Union and its strategic location on the persian Gulf, is a pivotal state in the East-West power balance. In the last decade, its has bought more than $10 billion worthof arms from the United States.

Without making firm predictions, Washington officials say that the removal of the Shah and the substitution of a fundamentalist Moslem regime would have serious consequences for the stability of that region.

The religious leaders are incapable of managing the process of modernization, in Washington's view, and the inevitable result would be the substitutionof a radical government - following the pattern of event sin Afghanistan.

The development would destablize the region and put heavy pressure on Saudi Arabia, whose support is necessary for the survival of the moderate Arab governments in the Middle East.

While asserting that the Soviets have an obvious interest in destabilizing the situation, these observers decline to make specific accusations Soviet involvement in the Iranian crisis.

As for the United States, the view appears to be that the American government can do little to aid the shah in what is essentially an internal crisis, but must make clear ita support for him and his government.

For now, in this view, the United States can only hope that the shah will be able to cope with the forces that are splintering his country.

In addition to the fighting at Paveh, major demonstrations were reported yesterday n the Tehran University campus and in the northeastern city Mashad, where 100,000 persons led by Shiite Moslems were said to have staged a protest.

The shah's harassed government was shaken internally when Mohammed Baheri, justice minister, and Manouchehr Azmun, minister of state for executive affairs, resigned from the Cabinet. They were the third and fourth ministers to leave the "national reconciliation" government formed by Prime Minister Jaafar Sharif-Emami on Aug. 27.