The Batticaloa District Peoples' Forum said on Monday it would honour Tamils killed by government soldiers and Muslim home guards since 1990.
The group, which is based in the island's east, said it would mark 5 September as an anniversary of the murder of 158 Tamil civilians at a refugee camp in 1990.
"It is our duty to remember those Tamil civilians killed by the Sinhalese army and Muslim home guards for more than two decades," the group said.
"Since June 1990 Tamil civilians have been abducted and killed in large numbers," it added.
The group said a foundation stone for a memorial tomb would be laid in Batticaloa, 230km east of Colombo, for the "more than 10,000 Tamil civilians killed since 1990".
The call for mourning comes after the government sent troops to the area, following a spate of killings of Muslims blamed on Tamils.
The Sri Lankan army has been
accused of widespread abuses
Sri Lanka's east, which is evenly split between Sinhalese, Tamils and Muslims, has regularly been a flashpoint during the island's 20-year civil war.
Reducing friction in the east is seen as one of the keys to a lasting peace in Sri Lanka, once stalled peace talks resume in the next few months.
Both the government and the Tamil Tigers have been accused of widespread human rights violations during the war in which an estimated 64,000 have been killed.
The rebels - accused of ethnic cleansing for evicting 100,000 Muslims from the north - are preparing a response to a government proposal on an interim administration for the north and east. The Tamils have been fighting the Sinhalese government for autonomy for 20 years.