Indonesia's top Islamic body on Tuesday condemned the sea burial of Osama bin Laden, as radicals promised a day of mass prayer to mourn the al Qaeda leader's death at the hands of US special forces.

President Susilo Bambang Yudhoyono's office issued a statement which neither applauded nor condemned the killing of the world's most wanted terrorist and alleged mastermind of the 9/11 attacks on the United States.

"The President said Indonesia has a common spirit with other nations in fighting against terrorism," spokesman Julian Pasha said in the first official Indonesian reaction to the news.

Indonesia is the world's most populous Muslim-majority nation and has been hit by multiple terror attacks including the 2002 Bali bombing which killed more than 200 people, mainly Western tourists.

Its US-backed counter-terrorism force has killed and arrested scores of Islamist militants since 9/11, including leaders of the Jemaah Islamiyah (JI) regional network who had links to al Qaeda.

But the Southeast Asian archipelago of 240 million people — 80 percent of whom are Muslims — remains home to a radical fringe with sympathies for bin Laden's campaign of global jihad, or "holy war", against the West.

The Police said they were on alert for possible retaliatory attacks.

"We are monitoring the situation and anticipating all possibilities," a top police officer told reporters.

Reflecting local sensitivities, the Indonesia Ulema Council (MUI), the country's highest Islamic body, slammed the US decision to bury bin Laden at sea.

"A Muslim, whatever his profession, even a criminal, their rites must be respected. There must be a prayer and the body should be wrapped in white cloth before being buried in the earth, not at sea," MUI chief Amidhan said.

"Many others have condemned it, especially as it was done with extraordinary hatred against him."

The United States says bin Laden received Muslim rites but his body was "eased" into the Arabian Sea so no one could turn his grave into a shrine. Other Muslim leaders have also condemned the method of burial.

Mahmud Azab, an adviser to Sheikh Ahmed al-Tayeb, the grand imam of Al-Azhar, the top Sunni Muslim authority in Cairo, said: "If it is true that the body was thrown into the sea, then Islam is totally against that.

"Any corpse, if it belongs to someone murdered or someone who died of natural causes, must be respected," he said, adding that Islam can only accept burials at sea if they are inevitable, for instance in a case of drowning.

A source close to the head of the Grand Mosque in Paris said a burial at sea "is totally against the sacrosanct rules of Islam".

In Jakarta, a violent vigilante group known as the Islamic Defenders Front announced plans to hold a "mass prayer for bin Laden" on Wednesday. Mourners were invited to "express gratitude to the late martyr Sheikh Osama bin Laden".

The group's Jakarta branch chairman, Habib Salim Alatas, said there was no proof that bin Laden was the mastermind of the September 11, 2001 attacks on New York and Washington that killed around 3,000 people.

"This is a question that must be answered by countries such as Australia, Britain, the United States and the Jews," he told AFP, adding that bin Laden was a "sincere fighter who defended Islam".

Yudhoyono's coalition government, which includes the conservative Muslim Prosperous Justice Party, has failed to ban such groups despite their frequent violent attacks on minorities and moderates.

Another radical Islamist group on Monday hailed the assassinated al Qaeda leader as a "martyr" who championed Islam.

"He fought for Islam and he fought for the lands colonised by America," Jemaah Ansharut Tauhid (JAT) spokesman Son Hadi told AFP.

"al Qaeda didn't die with him. Jihad will not be dampened just because he's dead, because jihad is a command of the religion, not of individuals."

JAT was founded in 2008 by firebrand cleric Abu Bakar Bashir, the alleged JI spiritual leader who is standing trial on charges of leading and financing another terrorist group.