Saudi authorities have arrested five of the country's best-known reformist intellectuals, sources told the BBC.
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.
Also arrested are two academics, Matrouq al-Faleh and Khalid al-Hameed, and two Islamists, Abdullah al-Hamid and Tawfiq al-Qaseer.
The last six months have seen rare displays of dissent
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.
Also, the country is under pressure from its ally, the United States, which ever since the attacks of the 11 September 2001 has been urging the Saudi rulers to take tougher action against extremism and intolerance.