* Political uncertainty hits Thai currency
* Protesters warned after blocking candidates
By Pairat Temphairojana
BANGKOK, Dec 23 (Reuters) - The Thai baht plumbed its lowest level in almost four years on Monday as a political crisis grew more intractable, with anti-government protesters attempting to stop candidates from registering for a February election.
Tens of thousands of protesters rallied across the capital on Sunday as they pressed ahead with their demands that Prime Minister Yingluck Shinawatra, the sister of self-exiled former premier Thaksin Shinawatra, resign.
Yingluck remains caretaker premier after calling a snap election for Feb. 2 in a bid to cool tension.
The main opposition party has decided to boycott the vote as it tries to stop her from renewing her mandate and perpetuating the influence of the powerful former telecommunications tycoon Thaksin.
It is an all-too-familiar picture after eight years of deadlock broadly between supporters and opponents of Thaksin, whose populist political machine has won every election since 2001 with widespread support from the rural poor in the north and northeast.
Opposed to Thaksin is the Bangkok-based establishment that backs protests against governments controlled by him.
Critics say Thaksin is a tax-dodging crony capitalist who wins elections with hand-outs, abused his power to enrich his family and friends and even tried to undermine the monarchy.
But to millions of farmers and other rural poor outside Bangkok, Thaksin is a benevolent billionaire who improved their living standards with cheap healthcare and state subsidies.
The seemingly irresolvable conflict hit the currency in Southeast Asia's second-biggest economy after rallies that left the outcome no clearer. The baht touched a low of 32.71 to the U.S. dollar, its weakest level since March 2010, according to Thomson Reuters data.
Satoshi Okagawa, a Singapore-based global markets analyst for Sumitomo Mitsui, said the baht had already been hit by the dollar's strength after the U.S. Federal Reserve decided last week to start trimming its bond-buying programme in January.
"There hasn't been any end to the political uncertainty," Okagawa said.
Thailand's future was muddied further at the weekend when the opposition Democrat Party decided to boycott the February election, saying the democratic system had been distorted by Thaksin and was failing the country.
The protesters are led by Suthep Thaugsuban, a firebrand former Democrat heavyweight whose campaign is less about policy than weakening the influence of the Shinawatra family before an election Yingluck's Puea Thai Party would most likely win.
He vowed on Sunday his supporters would stop election candidates from registering for the vote, a week-long process that began on Monday.
Several thousand protesters, watched by police and soldiers, began gathering soon after midnight around a Bangkok sports stadium where the registrations will take place.
By about 11 a.m. (0400 GMT), candidates representing 34 parties arrived to register. Nine were successful and the other 25 would lodge complaints with police, an Election Commission (EC) official said.
EC member Somchai Srisutthiyakorn told reporters the registration process could be extended beyond Friday's original closing date if not enough candidates had registered.
Police warned protesters they risked jail terms or stiff fines if they impeded the process.
The events of the past two days have only added to concern that Thailand could be left in political limbo. Suthep wants democracy to be suspended and for an appointed "people's council" to reform Thailand before any election can happen.
Yingluck had enjoyed a smooth two years in office but her party made a political miscalculation in November when it tried to push through an amnesty bill that would have nullified Thaksin's 2008 graft conviction, which he says was politically motivated, and allowed him to return home from self-exile.
The protests, which have attracted up to 160,00 people, have been largely peaceful but have failed to stop Yingluck's government from functioning.
Government supporters have stayed away but are likely to be enraged if they see Yingluck forced from office.
The Democrats boycotted an election called during similar protests in 2006, when Thaksin tried to renew his mandate. His party won, but the result was annulled on a technicality. (Additional reporting by Panarat Thepgumpanat, Masayuki Kitano in SINGAPORE; Writing by Paul Tait; Editing by Robert Birsel)