Indonesia's military commander in Aceh said the crowd gathered after residents accused the monitors, from a security committee supervising a December 9 peace deal, of failing to respond to complaints against Free Aceh Movement (GAM) rebels.
Officials from the Henry Dunant Centre (HDC), which brokered the landmark pact between the rebels and Jakarta, said two of the eight monitors had been slightly hurt in the incident in central Aceh, which lasted more than two hours.
They said the two were a government soldier and a rebel representative. Parts of the office in Takengon town were ransacked in the first attack on any monitors since they spread out through the province, they said.
David Gorman, Indonesia head of the Geneva-based HDC, condemned the incident.
"We strongly condemn this type of act. We take any matters that threaten the monitors, extremely seriously," he said.
It was not clear if all eight were in the office at the time, or if the group would remain in the town.
There were three Thai monitors, three from GAM and two from the government, HDC officials said.
Aceh military chief Major General Djali Yusuf said the residents had alleged a coffee farmer had been kidnapped by the rebels, forcing them to cough up ransom money. "The people reported this to the JSC, but there was no response. Finally they got angry and wanted to chase the JSC out, but the situation is now calm," Yusuf said, referring to the joint security committee.
HDC officials said most of the anger appeared to be directed at the GAM and government representatives.
Diplomats have said the December 9 agreement was the best chance in decades to bring lasting peace to Aceh, although there have been a number of violations blamed on both sides and some government officials have warned the pact could be in danger.
Aceh is 1,700 km (1,060 miles) northwest of Jakarta on the tip of Sumatra island. It is one of Indonesia's two separatist hot spots. The other is Papua in the east.
In the two years leading up to the pact an estimated 4,000 people -- civilians, government troops and rebels -- were killed in the long-running conflict in the oil and gas-rich province.
Since the pact, violence has dropped markedly.
(With reporting by Telly Nathalia)
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