By Roger Hardy BBC's Middle East analyst

Those arrested are both liberal and Islamist figures who have put their names to petitions calling for wide-ranging political and economic reform.

They include publisher Mohammed Said Tayib - one of the prime movers organising petitions calling on the House of Saud to accelerate reform.

There has been no official confirmation of the arrests.

SIGNS OF DISSENT September Saudi intellectuals launch reform petition 13 October First council elections promised 14 October Rare protest in Riyadh calling for reform 9 November Suicide bomber kills 17 in a housing compound December New petition

Also arrested are two academics, Matrouq al-Faleh and Khalid al-Hameed, and two Islamists, Abdullah al-Hamid and Tawfiq al-Qaseer.

Saudi analysts say the arrests may be a warning-shot designed to deter the liberals at a time when the ruling princes find themselves under an unprecedented degree of pressure.

More than 800 liberal reformers signed a petition only last month calling for an elected parliament and a bigger role for women.

Meanwhile, religious conservatives are anxious to preserve the status of Wahhabism, the country's austere brand of Sunni Islam.