King Abdullah had been de facto ruler for 10 years

The grand mufti was among the first to file past the king in the investiture ceremony at a Riyadh palace.

In a televised speech afterwards, King Abdullah promised he would work for justice and serve his people.

Western leaders, including Britain's Tony Blair and US Vice-President Dick Cheney, also paid their respects.

The ceremony comes a day after the funeral of King Fahd.

Everyone taking part in the formal investiture - an Islamic tradition known as "bayaa" - had to shake the hand of the new king and swear an oath of allegiance to him.

I am assuming responsibility after the death of the dear one and I feel that the burden is heavy and that the trust is great

King Abdullah

Some shouted "Long live the king" as a palace servant moved among the crowd, swinging an incense burner to bless them, the Associated Press reports.

Although the ceremony is supposed to be open to all citizens, security was tight with surrounding neighbourhoods closed off, checkpoints set up and ID cards checked.

Armed security officials wandered around the grand hall where the ceremony was taking place.

Mr Blair and French President Jacques Chirac joined European royalty at the ceremony.

Mr Cheney, along with former President George Bush Snr, were also flying into Riyadh - a reflection of the close ties that the house of Saud maintains with the West on many levels, a BBC correspondent says.

'Proving critics wrong'

In his speech after the ceremony, King Abdullah promised to rule with righteousness and justice and, "to serve all people without discrimination... I ask you to be my strength and that you support me."

"I am assuming responsibility after the death of the dear one and I feel that the burden is heavy and that the trust is great," he added.

With the "bayaa" ceremony, the 81-year-old Abdullah bin Abdul-Aziz gains full legitimacy according to Saudi Arabia's highly conservative Islamic system.

As crown prince, he had been the de facto ruler of Saudi Arabia for the last 10 years, since King Fahd, his half-brother, suffered a stroke in 1995.

Newspapers in the kingdom have again been full of eulogies for the late king and vows of loyalty to King Abdullah.

Following Saudi custom, King Fahd - one of the world's richest monarchs - was buried in an unmarked plot in a public cemetery on Tuesday.

Under Saudi Arabia's Islamic system, there was no public mourning for the late king, although other Arab and Muslim countries have declared official periods of mourning.