Political officer Mahendra Ratnaweera was severely injured in the attack while his passenger, freelance journalist Namal Perera, also suffered serious injuries.
Perera told reporters from his hospital bed that he locked himself in the car but the attackers smashed windows.
"They continued to beat us with batons and clubs and escaped when a large group of bystanders gathered," said Perera, who had been a defence analyst for a private newspaper.
Both were admitted to a nearby private hospital and received treatment.
The attack came just six days after Sri Lanka's government moved to deflect a barrage of criticism over its press rights record by promising to help journalists receiving death threats.
"This is a very serious matter and we will take all steps to get at the perpetrators," Media Minister Anura Yapa told AFP.
British High Commissioner (ambassador) Peter Hayes condemned the attack and called on authorities to thoroughly investigate the incident and bring the cul