Police officials said these arrests were made from a series of search operations around the Colombo eastern suburb of Nugegoda where the blast occurred.
Police have traced a parcel bomb put inside a popular department store at Nugegoda for the explosion and blamed the rebel Liberation Tigers of Tamil Eelam (LTTE) for the attack.
Four separate police teams have been deployed to carry out investigations.
Wednesday evening's bomb attack came just 10 hours after the rebels making an attempt on the life of Tamil politician Douglas Devananda, the minister of Social Services.
Police spokesman Jayantha Wickramaratne on Thursday called for stringent public vigil for suspicious people, packages and movements, adding that special police security operations have been launched to prevent similar incidents.
The Sri Lankan parliament observed a minute's silence to pay respect to the bomb victims Thursday morning.
The main left party, the JVP or the People's Liberation Front called for the banning of the LTTE for targeting civilians through bomb attacks.
The police on Thursday carried out a search operation in a popular shopping mall named Liberty Plaza in Colombo following a telephone call saying a bomb had been taken into the building.
However, a joint police and Army search did not find any explosives, officials said.
As a security measure, all vehicles have been banned from entering into Colombo's business landmark, the World Trade Center twin towers in the business district of Fort.
Analysts say that the city of Colombo is facing the effects of violence that escalated since the end of 2005.
More than 5000 people have died in the new cycle of violence, making the Norwegian-brokered ceasefire agreement exist only on paper.
Claiming discrimination at the hands of the Sinhala majority, the LTTE has been fighting the government since the mid-1980s to establish a separate homeland for the minority Tamils in the north and east.
Güncelleme Tarihi: 29 Kasım 2007, 13:53