KUWAIT (Reuters) - Kuwaiti authorities arrested an opposition leader late on Monday after he made comments viewed as critical of the Gulf Arab state's ruler, a political activist said.

Musallam al-Barrak, a former lawmaker and a prominent figure in the nationalist Popular Action Bloc, was arrested on Monday night following a news conference at his house where he called on the government to abide by the constitution.

His arrest seems likely to fuel political tensions in the oil exporting state, which has seen some violent protests in recent weeks following changes to the country's electoral law that critics say will hamper the opposition in a parliamentary ballot scheduled for December 1.

At an opposition-led rally on October 15 where Kuwaiti civilians clashed with riot police, Barrak appealed to Emir Sheikh Sabah al-Ahmad al-Sabah to avoid "autocratic rule".

Although Kuwait allows more freedom of speech than some other Gulf states, the emir, who has the last say in state affairs, is considered "immune and inviolable" in the constitution.

"Members of the state security service came in with an arrest warrant and he (Barrak) is now being held at the state security department," activist Ahmad al-Dayeen told Reuters.

He described the arrest as a serious development that could have an impact on protest rallies scheduled for November 4.

With the uprisings which have swept through much of the Arab world aggravating tensions between Kuwait's elected parliament and a government dominated by the Al-Sabah ruling family, the OPEC member state has shown limited tolerance for dissent.

Last week, authorities released three former opposition lawmakers accused of criticising the emir and four people arrested for taking part in protests this month. A court hearing for the former MPs was set for November 13.

Tens of thousands protested the electoral law reforms, which were ordered by the emir, leading to some of the worst violence in the country's recent history.

The opposition has said it will boycott the December 1 election.

Kuwait's oil wealth and generous welfare state has helped it avoid the kind of uprisings that toppled leaders elsewhere in the region.

On Monday, credit agency Fitch warned Kuwait on that further escalation of political protests could put its AA sovereign rating under pressure despite the nation's strong balance sheet.

(Reporting by Ahmed Hagagy; Writing by Rania El Gamal; Editing by John Stonestreet)