Gulf stock markets suffered broad sell-offs on Tuesday as Brent crude oil slumped to a fresh 5-1/2-year low and political uncertainty in Greece pressured global equities.

Brent fell as low as $56.74 per barrel, its lowest since May 2009, as persistent worries about a global supply glut offset concerns about output disruptions in Libya.

European stock markets, to which Gulf investors often turn for cues, were down after Greek Prime Minister Antonis Samaras failed to get enough support for his presidential nominee; he will call a national election that could threaten Greece's euro zone bailout.

Saudi Arabia's main stock index tumbled 4.2 percent in active trade. Petrochemicals giant Saudi Basic Industries , down 4.6 percent, was the main drag. Overall, the Saudi petrochemicals sector lost 5.0 percent.

Prices of petrochemicals are often linked to crude prices and some Gulf producers have enjoyed access to subsidised feedstock which gave them better margins. With cheap oil, that comparative advantage has been slashed.

Another reason for markets' sharp drop was that many institutions such as fund management firms are almost inactive during the Christmas and New Year holidays, leaving stocks at the mercy of volatile retail investors.

However, Tuesday's declines were not as huge as those seen in mid-December, when Saudi Arabia's benchmark lost 7.3 percent in a single day in response to oil's weakness.

The kingdom's government last week published a 2015 budget that will maintain spending at a high level, easing some investors' fears about expenditure cuts which could hurt economic growth and corporate profits.

UAE, KUWAIT

Dubai's index tumbled 5.4 percent. Builder Arabtec Holding and developer Emaar Properties, the most heavily traded stocks, lost 9.1 and 6.5 percent respectively.

Some stocks, such as property firm Deyaar and builder Drake and Scull, fell by their daily 10 percent limits.

Although Dubai is much less dependent on oil revenues than other Gulf states, its stock market has been hit hardest in the region in recent months because of its higher liquidity, exposure to foreign investors and the unwinding of leveraged positions.

According to bourse data, retail investors accounted for 86 percent of selling activity on Tuesday and foreigners were also net sellers.

On other Gulf markets, Abu Dhabi's index dropped 2.2 percent, Oman lost 2.7 percent and Qatar's bourse was down 1.9 percent.

Kuwait's index fell 1.8 percent, but most of the day's trading was in the shares of Boubyan Bank, which added 1.3 percent to 0.405 dinar. A total of 94.2 million shares representing a 4.8 percent stake in the bank changed hands on Tuesday.

Elham Yousry Mahfouz, chief executive of Commercial Bank of Kuwait (CBK) which is one of Boubyan's main shareholders, told Reuters that CBK had reduced its stake by selling shares on the market in order to boost its capital adequacy.

CBK had a 17.7 percent stake in Boubyan as of Dec. 29, according to bourse data. CBK shares climbed 1.6 percent on Tuesday.

Outside the Gulf, Egypt's bourse edged up 0.5 percent. The Cairo government signed a deal on Monday to import liquefied natural gas from Algeria between April and September, a move aimed at easing a chronic energy shortage.