At least 1,200 officers will be deployed for the 14-hour rally in the retail heart of the city, where the 2010 protests in support of ousted premier Thaksin Shinawatra descended into the kingdom's worst violence in decades.
Police blocked traffic at the Ratchaprasong intersection, one of Bangkok's busiest junctions, as busloads of Red Shirts from across the country arrived in the city centre a few hours ahead of the rally.
"We expect tens of thousands of them to join the anniversary," Major General Worasak Nopsitthiporn, deputy commander of Bangkok Metropolitan Police Bureau, said.
"Traffic in front of Central World (a landmark shopping mall) is blocked after they set up the rally stage last night."
Red Shirt leaders expect between 100,000 and 200,000 people at the rally, which will begin with Buddhist prayers for those killed in the 2010 unrest and see a live video link address from Red Shirt hero Thaksin, who lives overseas to avoid arrest in Thailand.
More than 90 people, mostly civilians, died in the 2010 violence, which marked the culmination of a series of rival protests since a 2006 coup that toppled Thaksin.
The Red Shirts and rights groups have called on the new government, led by Thaksin's sister Yingluck, to prosecute soldiers and officials responsible for causing the deaths and injuries, many to unarmed demonstrators.
"We have to seek justice otherwise the use of force to crackdown on protesters may happen again," Red Shirt leader Thida Thavorseth said Friday.
Thailand has seen a string of violent protests since 2005, involving the arch royalist and nationalist Yellow Shirts, the mainly rural working-class Red Shirts and several smaller factions.