BANGKOK, April 11, 2011 (AFP) - Thailand should allow foreign observers to monitor its upcoming election, which could trigger fresh political violence and mass protests, an international think-tank said Monday.

"In the interests of enhancing the legitimacy of the polls, all sides should support election observation," International Crisis Group (ICG) said in a report titled "Thailand: The Calm Before Another Storm?"

"Xenophobic reaction to electoral observation by foreigners is counter-productive," it added.

Thai Deputy Prime Minister Suthep Thaugsuban said last month international poll observers would be "inappropriate" and that he did "not respect" Westerners.

ICG urged the country's election commission not to block international monitoring as it did during the previous election, when the European Union had offered in vain to deploy observers.

Thailand has never officially allowed monitors to operate in polling stations during its elections, but the Asian Network for Free Elections (ANFREL) has been allowed to work in the country during the last two votes.

The "Red Shirt" opposition movement that was behind mass street protests in Bangkok last year has called for international observers to be present at the upcoming election, which is expected in late June or early July.

Thai society remains deeply split a year after mass demonstrations by the Red Shirts descended into violence which left more than 90 people dead -- mostly civilians -- in a series of clashes between protesters and armed troops.

It was the worst political violence in decades, and the ICG report warned the polls could bring fresh conflict.

"As the stakes are high, the forthcoming elections could be violent," it said, noting that one local politician was killed and two others seriously injured in three separate attacks on March 2, 2011.

"While the initial investigation indicated that the attacks were likely to be linked to local rather than national politics, these incidents showed some may use violence to settle their disputes or eliminate their political rivals," it said.

Commentators believe the upcoming poll is likely to highlight the political tensions that have beset the kingdom since the Reds' hero, fugitive former premier Thaksin Shinawatra, was ousted in 2006 in a military coup.

The movement views the current army-backed government of Prime Minister Abhisit Vejjajiva as undemocratic because it came to power in a 2008 parliamentary vote after a court ruling threw out the previous administration.

"Anger or any sense of injustice over how the new government is formed could be the rallying cry for renewed mass protests," ICG warned