Bangkok, Thailand (CNN) -- Anti-government protesters gathered at a Bangkok prison on Friday and demanded the release of their leaders, as well as all other political prisoners.
The demonstrators laid red roses in front of Bangkok Remand Prison to offer their detained leaders moral support.
Protesters are expected to converge in the capital on Sunday, the fourth anniversary of the bloodless overthrow of Prime Minister Thaksin Shinawatra. Authorities are on alert, but are prepared to control the situation, Deputy Prime Minister Suthep Thaugsuban said on the Thai television network MCOT.
Violence broke out during anti-government protests in Bangkok from March through May, leaving 91 dead and more than 2,000 injured.
"Red Shirt" demonstrators, known for their clothing, mobilized to seize the Ratchaprasong and Phan Fa areas in March. Ratchaprasong later became the main protest venue, while other parts of Bangkok remained peaceful. The violence crippled hotels and luxury shops, and badly damaged Thailand's tourism industry.
Sunday's demonstration will start with the release of red balloons, said Red Shirt organizer Sombat Boon-ngam-anong. The government has not explained the 91 deaths from March through May, so protesters will write letters to the sky, as a last resort to learn the truth, he said. Demonstrators also will tie 100,000 pieces of red cloth around Ratchprasong area; lie down as if they were dead; and dress as ghosts.
Sombat estimated that 5,000 people will take part in Sunday's demonstration in Bangkok.
Another protest is expected in Chiang Mai, in the north, a Red Shirt stronghold.
Bangkok and six other provinces remain under emergency rule, which the government imposed in April. The decree allows authorities to detain suspects without formal charges up to three months.
The Red Shirts support Thaksin, who was ousted in a bloodless military coup in 2006 while he was attending a U.N. assembly. His supporters protested for weeks, demanding that Prime Minister Abhisit Vejjajiva dissolve parliament and call new elections.
Thaksin briefly returned to Thailand to fight a corruption charge, but fled again in 2008. He left before Thailand's Supreme Court convicted him of abusing his power and purchasing land owned by his wife.