At least 1,200 officers will be deployed Saturday in the retail heart of Bangkok, authorities said, where the 2010 protests in support of ousted premier Thaksin Shinawatra descended into the kingdom's worst violence in decades.
"We expect a huge turnout but at this stage I have no estimate of the numbers," said police spokesman Colonel Preeda Sataworn.
Red Shirt leader Thida Thavornseth said more than 100,000 people from across the country could converge on Bangkok for the 12-hour gathering at the Ratchaprasong intersection, close to upmarket hotels and shopping malls.
More than 90 people, mostly civilians, were killed and nearly 1,900 wounded during the two months of rallies, which ended on May 19 2010 after the government of then premier Abhisit Vejjajiva sent in armed troops.
The Red Shirts have called on the new government, led by Thaksin's sister Yingluck, to prosecute soldiers and officials responsible for causing scores of deaths and hundreds of injuries, many to unarmed demonstrators.
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.
Rights groups say the military -- and some armed elements within the Red Shirts -- have so far evaded justice for their roles in the violence, causing anguish to the victims' families.
"The government needs to prosecute all those responsible for crimes, whatever their political affiliation or official position, to end the cycle of violence and impunity," said Brad Adams, Asia director for Human Rights Watch.
Yingluck's government came to power last year with a promise of reconciliation in the deeply divided nation.
Her government in January agreed a 2.0 billion baht ($65 million) pot to compensate all victims of political unrest in recent years.
But criminal investigations into the 2010 deaths have yet to lead to any prosecutions and the government is debating plans to offer a sweeping amnesty for the violence as part of reconciliation efforts.
The opposition says the proposed amnesty aims to bring home Thaksin, who lives abroad, to avoid a jail term for corruption. He also faces terrorism charges relating to the 2010 violence.
Families of the victims have also decried the idea of an amnesty.
Nutthaputt Akahad, the brother of a nurse who was gunned down in the grounds of a Buddhist temple while treating injured Red Shirts on May 19, said Thailand risked becoming a "lawless country" unless the killers are prosecuted.
"We have sacrificed our loved ones, some have sacrificed their freedom, but we can't sacrifice the truth, we can't sacrifice justice," he said at an event on Tuesday in the run-up to the anniversary.