ISLAMABAD, PAKISTAN, SEPT. 30 -- Afghan leader Najibullah tightened his grip on power today by taking over as head of state, a post due to have sweeping powers in the future.

The official Kabul Radio said he was unanimously elected president of the legislative Revolutionary Council. This post carries with it the national presidency.

A new constitution being drawn up by the Soviet-backed government would greatly expand the powers of the president, currently largely a figurehead position.

He will have the authority to dissolve a national assembly and veto its laws, declare war or states of emergency, appoint the prime minister, top military and judicial officials and command the armed forces.

Western diplomats here said the move clearly confirmed the former secret police chief as Afghan strongman. It also indicated that a long-awaited second conference of his ruling People's Democratic Party of Afghanistan was imminent, to debate the new constitution.

In an acceptance speech reported by the official Bakhtar news agency, Najibullah said that a gathering of tribal elders, ministers and clergymen would be convened soon to approve the new constitution.

A national assembly, which will take over legislative functions from the Revolutionary Council, will also be elected.

Bakhtar also announced that the new head of state, who has recently called himself simply Najib, was restoring the Islamic element in his name and will again officially be called Najibullah.

Najibullah replaced caretaker president Mohammed Tsamkani, who reverts to first vice president of the council's Presidium.