BANGKOK - Thousands of supporters of former Thai premier Thaksin Shinawatra rallied in Bangkok on Thursday, turning up the heat on Thailand's embattled government ahead of big protests planned for January in a push for new elections.

The United Front for Democracy Against Dictatorship (UDD) held a one-day rally near Democracy Monument to press for the dissolution of parliament and a repeal of the 2007 constitution, drafted by the army after it ousted Thaksin in a coup in 2006.

The UDD and its red-shirted supporters say that constitution is undemocratic and want a return to the constitution under which Thaksin won two landslide elections in 2001 and 2005. Thursday was a public holiday in Thailand for Constitution Day.

During the night the exiled Thaksin phoned his supporters at the rally in the Thai capital, who numbered 30,000 by then, according to protest organisers.

Our red-shirt group wants to bring the 1997 constitution back, organisers quoted him as saying, adding that democracy in Thailand was paralysed and weak.

The UDD had initially planned prolonged street rallies for this month, rattling financial markets, but they were postponed out of respect for revered King Bhumibol Adulyadej and the festivities to mark his 82nd birthday on December 5.

UDD opponents have accused the red shirts and Thaksin of pursuing a republican agenda, which they deny.

We are here today to request a change in the constitution because the existing one causes problems to the country. We reassure everyone that our rally will be peaceful and we have no weapons, Jatuporn Phrompan, a UDD coordinator, told Reuters.

Our next mission is to expel the government, he added, declining to say precisely when larger-scale protests might start in the new year.
The extra-parliamentary UDD says it is a pro-democracy movement battling Thailand's traditional power-holders -- royalists, urban business elites and the military -- whom they accuse of undermining elected, pro-Thaksin governments.

Thaksin was sentenced in absentia to two years in prison in 2008 for conflict of interest offences while in office. He is in self-imposed exile, based in Dubai.

Thursday's red shirt rally was the first since September that had gone ahead without the government invoking a harsh security law allowing the military to control demonstrations. The UDD said it had 1,000 of its own guards to prevent any unrest.

Pro-Thaksin rallies have played a major part in Thailand's protracted turmoil this year, hampering the government's ability to run the country and prompting fears of a violent escalation.

Credit ratings for Thailand, Southeast Asia's second-biggest economy, have been downgraded as a result of the instability, and worries are growing about the major rallies planned for January.

(Writing by Martin Petty and Khettiya Jittapong; Editing by Alan Raybould and Paul Tait)