The demonstrators, led by fiery former deputy premier Suthep Thaugsuban, have vowed to derail the ballot and demand instead an appointed "people's council" before a future vote.
Suthep has vowed to seize ministries and other sites across the capital, although it is not clear when that will start. The main opposition Democrat Party has also declared it will boycott the election.
Thailand's Electoral Commission has offered to act as a mediator between Puea Thai, the Democrats and the protesters. Commissioner Somchai Srisuthiyakorn said the commission would meet senior members of Puea Thai and the Democrats on Thursday, although he said the protesters had rejected a similar offer.
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"I believe that something positive will come out of the meeting and the situation will ease up," Thai media quoted Somchai as saying.
While the protests have mainly been in Bangkok, election registration has also been blocked in at least seven provinces in the south, where the protesters and Democrats draw support.
The wider aim of the protesters is to neutralize the power of Thaksin, who they say has manipulated democracy by buying the support of the rural poor with populist policies such as cheap healthcare and subsidies for rice farmers.
(Read more: Spotlight on Thailand as political strife escalates)
Thaksin was overthrown in a 2006 coup and fled into exile two years later to avoid jail for graft charges he said were politically motivated. In November, Puea Thai tried to push through an amnesty bill that would have allowed Thaksin to return a free man, sparking the latest round of protests.