The attack came early on Saturday just hours after the Sri Lanka Monitoring Mission (SLMM) reprimanded both Liberation Tigers of Tamil Eelam separatists and the government over a spike in violence.
It was the first attack against the monitors since a 2002 ceasefire halted a two-decade civil war.
The bomb ripped through a car belonging to Scandinavian truce monitors, but there were no casualties, police said.
The pickup truck of the Norwegian-led Sri Lanka Monitoring Mission (SLMM) was parked in their own premises in the town of Batticaloa, 300km east of Colombo, when the bomb went off, police said.
Helen Olafsdottir, spokeswoman for the SLMM, said: "A hand grenade was lobbed into our compound in Batticaloa and exploded. We don't know who was behind it. It is the first time one of our offices has been directly attacked."
A police official in Batticaloa, said: "We have begun an investigation and the military has also started checking the other vehicles there to see if bombs have been planted in them too."
There was no immediate claim of responsibility.
Saturday's grenade attack took
place in Batticaloa in the east
Some 60 unarmed monitors, all from Nordic countries, have been there since 2002.
On Friday, they questioned whether there was still a ceasefire to monitor after more than 100 people were killed in December in attacks that have continued into January.
Fears of a return to a war that killed more than 64,000 people before the truce are growing in tandem with deadly attacks by suspected rebels on the military and reported abuses by the armed forces against civilians.
Suspected Tigers killed nine sailors in an ambush on a main supply route in Sri Lanka's north on Thursday. It came after similar ambushes killed 39 military personnel in December.
Analysts are sceptical of rebel denials of involvement in the attacks, but say evidence suggests elements in the military are hitting back either directly or by helping renegade rebels target the Tigers, despite their denials.
Diplomats and defence experts say both sides are engaged in an undeclared war that has hammered the stock market and could spiral into an all-out conflict that would choke a $20 billion economy dependent on textile and tea exports and tourism.
"Killings and serious attacks continue and the situation is getting worse"
Sri Lanka Monitoring Mission statement
The SLMM warned on Friday in a statement: "Killings and serious attacks continue and the situation is getting worse."
The Sri Lankan government has repeatedly said it will not be provoked into resuming war, but Mahinda Rajapakse, the new president, is having to juggle the demands of hardline allies who hate the Tigers and an increasingly frustrated military.
The Tigers have threatened to resume their armed struggle this year unless the government gives them a separate Tamil homeland and wide political powers in the north and east, where they already run a de facto state.
The rebels say they want lasting peace, but they also say they are ready to use suicide bombers as in the past if war resumes.