Following protests outside the embassy on Sunday, an unknown number of protesters occupied the building for several minutes, Even Joerstad, an Oslo police superintendent, said.
"There were about 100 outside but we don't know how many went inside the embassy," he said.
"They were inside for about four or five minutes and then they went out."
At the request of the Sri Lankan ambassador, Norway's foreign ministry has asked Oslo's police to strengthen surveillance of the Sri Lankan embassy.
"I condemn the illegal intrusion of outside persons into the embassy of Sri Lanka and the significant damage they did," Jonas Gahr Stoere, Norway's foreign minister, said in a statement.
Supporters and Tamil expatriates have rallied for several days in various European capitals to protest against the Sri Lankan military offensive against the Tamil Tigers
More than 100,000 people marched in London, the British capital, on Saturday to demand a ceasefire.
In 2002, Oslo negotiated a ceasefire between the two sides, but it collapsed only three years later when Rakapaksa came to power, relaunching the 25-year separatist war.
In Sri Lanka, on Sunday, the government declared it would institute a two-day lull in the fighting to allow tens of thousands of civilians trapped in the conflict zone to flee to safety.