The policeman were guarding the Eelam People's Democratic Party's office in Jaffna, 300 kilometers (185 miles) north of Colombo, the Sri Lankan capital, when they were attacked late Wednesday, the government's national security Web site said.
The suspected rebels used small arms and grenades for the attack. The wounded policemen have been admitted to a local hospital.
The party, better known by its acronym, EPDP, is a coalition partner in the Sri Lankan government. The party in the past had been targeted by Tamil Tiger rebels, their bitter rivals.
The EPDP used to be a militant group but it renounced violence and joined mainstream Sri Lankan politics a decade ago. The party accuses the rebels of killing at least 40 of its members since a cease-fire between the rebels and the government came into effect in 2002, the AP reports.
Tamil rebels have been fighting since 1983 for a separate homeland for minority ethnic Tamils.