Norway's governing Labor Party has tightened security at its first convention since a far-right extremist who railed against the party's immigration policies killed 77 people in a bombing and shooting massacre.

Labor officials cited an "overall assessment" of the situation in Norway, but wouldn't say whether the raised security level was linked to Anders Behring Breivik's attacks on July 22, 2011, or the explosions at the Boston Marathon this week.

The four-day convention starts Thursday with Prime Minister Jens Stoltenberg leading a memorial service for the victims of Breivik's violence.

The anti-Muslim fanatic detonated a bomb that killed eight in Oslo before killing 69 people, mostly teenagers, at the Labor Party's youth summer camp. He's serving a 21-year sentence that can be extended for as long as he's considered dangerous.