"The arrest of Dushantha Basnayake, who is close to a political adversary of President Mahinda Rajapakse, looks above all like retaliation," RSF said in a statement.
"The fears of human rights organisations have unfortunately turned out to be well-founded: The anti-terror law is being misused to silence critics. We call on the Sri Lankan government to release Dushantha Basnayake," it said.
Dushantha Basnayake, aged 40 and the father of two children, is Sinhalese. The weekly Mawbima has a reputation for criticising the government and for condemning human rights abuses and corruption in Sri Lanka, RSF said.
The newspaper is owned by close ally of former Sri Lanka foreign minister Mangala Samaraweera who was sacked from the government together with two others earlier in February.
"Officers of the