More than 1,000 vehicles filled with protesters have fanned out across Thailand's capital to enlist residents for their anti-government campaign.
They plan to follow up the march on Saturday with a "blood painting", the latest shock tactic by demonstrators in their weeklong campaign to oust Abhisit Vejjajiva, the country's prime minister.
The protesters plan to drive 70km through Bangkok, setting off from the centre of the city and driving through the central business district, Chinatown and outlying residential areas.
City and police officials urged residents to use public transport and either stay at home or at their workplaces until the demonstration ends.
The protesters want Abhisit, who they accuse of taking power through illegitimate means, to dissolve parliament and call fresh elections - a demand he has repeatedly rejected.
In an attempt to dramatise their demands, thousands of so-called red shirt demonstrators lined up on Tuesday to donate blood to their cause.
Leaders claimed they collected 300,000 cubic cm of blood that were transferred into dozens of large plastic jugs.
Most of the blood was splattered at Abhisit's office, at the headquarters of his ruling party and at his private residence.
Protest leaders say they have 15 jugs of blood left and plan to use it to create a massive work of art.
Jatuporn Prompan, a protest leader, said: "Artists and red shirts will be invited to partake in a blood painting."
They plan to unfurl a giant white cloth on which supporters will be invited to paint pictures, scrawl poems and express political statements.
"The theme of this artwork will be the history of the people's fight for democracy," Jatuporn said.
The red shirts describe their struggle as one between "phrai", the common people, and "amataya", upper-class bureaucrats and other members of the elite.
The group largely consists of supporters of Thaksin Shinawatra, the former prime minister, who was ousted by a 2006 military coup for alleged corruption, and pro-democracy activists who opposed the army takeover.
Thaksin lives in self-imposed exile, mostly in Dubai, and has delivered several video-link addresses to the red shirts.
The red shirts brought him two landslide election wins and remain loyal because of his populist policies.
"Fellow Bangkokians, send any kind of signals, wave a red flag, give some water, so that our red shirts can feel at ease," Thaksin urged in a video link-up on Friday.
"I apologise for the traffic congestion lately and there will be more traffic jams when we march.
"I apologise. I owe you one.
"When I return, [I promise] 10 lines of electric trains from Bangkok to the surrounding provinces."