Saudi Arabian authorities have uncovered "terrorist" cells plotting attacks against police and civilians in the capital Riyadh, a news agency reports.

The Saudi Press Agency quoted an unidentified interior ministry official as saying that six Yemenis have been arrested after they prepared explosives and experimented with them outside Riyadh. Explosives were seized, the report said.

The official referred to the suspected terrorists as belonging to a "deviant group", a common Saudi description of al Qaida.

"The terrorist cell in Riyadh promoted the deviant Takfiri thought," the statement said, in reference to an al Qaida-like extremist ideology.

The report said they were connected to "suspected elements" inside and outside the kingdom.

Saudi Arabia has been cracking down on Islamic militants since al Qaida launched a wave of attacks in the country in 2003, killing dozens.