P Uthayakumar's lawyer M Manoharan said the appeal was filed at the Session Court yesterday.
Uthayakumar, 50, was sentenced to two years six months in prison on June 5 for writing an open letter in 2007 addressed to then British Prime Minister Gordon Brown seeking his help to protect ethnic-Indians in the country.
Sessions Court Judge Ahmad Zamzani Mohd Zain ruled that Hindu Rights Action Force (Hindraf) founder Uthayakumar had failed to raise reasonable doubt.
"Would you like to say anything in mitigation, though you have so far taken the stand not to submit, in protest," the judge asked Uthayakumar before handing out the sentence.
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Uthayakumar had refused to submit a defence or mitigation, claiming that it was in protest over how Indians in Malaysia were being treated.
However, his Counsel Manoharan told reporters Uthayakumar had decided to appeal after he advised him to do so, and the appeal was filed at the court registry yesterday, The Star online reported.
The former Internal Security Act detainee was charged on December 11, 2007 with publishing a seditious letter on the Police Watch Malaysia website addressed to Brown.
He had made the declaratory application (to declare the Sedition Act unconstitutional), in a bid to have the charge against him under the Act to be revoked and the prosecution in the proceedings in the Sessions Court be struck out.