Jakarta – Thousands of white-clad, religious conservatives rallied and prayed in cities across Indonesia on Sunday, demanding the imposition of traditional Islamic law in the world's most populous Muslim nation.

Organisers said 20,000 supporters gathered in several cities, but police and witnesses said only about 2,000 marched in the capital, Jakarta, and a few hundred in Surabaya, Indonesia's second-largest city.

The rallies were part of a largely unsuccessful campaign to convince the country's Muslim majority to embrace Syariah or Islamic law. Although more than 80 per cent of the country's 210 million people are Muslim, only the war-torn province of Aceh has implemented the system on a small scale.

Syariah law is derived from the sayings of the Prophet Muhammed, Islamic tradition and the the Quran. It is a wide-ranging system that regulates many aspects of public and personal life.

Women wearing headscarves and men dressed in long, white robes were among the demonstrators who marched through central Jakarta on Sunday. They carried banners reading, "Uphold Syariah" and chanted "Allahu akbar" or "God Is Great".

Some speakers urged supporters to vote only for candidates who support Syariah in the April 5 parliamentary elections. Others turned the event into a religious gathering, leading the crowd in chanting passages from the Quran.

"If you are Muslim, you have to struggle to establish Syariah law," said Harimoekti, an activist with the conservative non-governmental organisation Hizbut Tahrir Indonesia, or Indonesian Liberation, which helped to organise the rallies.

"Under Syariah law, we can prevent corruption and improve the daily lives of people," he said. "The world would be a beautiful place with Syariah."

In Surabaya, the crowd marched to local government offices calling for the removal of anyone without sufficient Islamic credentials.

Indonesia's founding fathers wrote a constitution in 1945 for a secular government and religious tolerance between the Muslim majority and Christian, Buddhist, Hindu and other minorities. Successive governments have fended off calls for Indonesia to become an Islamic state. In 2002, lawmakers rejected calls to amend the constitution to include Syariah law and the country's largest Muslim groups have repeatedly opposed making it state policy.