Buddhist-led mobs tore through streets hurling stones at the offices and residences of international aid workers in Myanmar's western Rakhine state on Thursday (local time), prompting the evacuation of almost all non-essential staff, residents and officials said.
Some were flown out, others placed under protection at a police guest house.
There were no immediate indications anyone was hurt in the violence, which started in the state capital, Sittwe, late Wednesday and picked up again early Thursday, with angry crowds swelling in size from several hundred to more than 1000.
At least one building was looted and three cars damaged, aid workers said on condition of anonymity because they feared retaliation.
State-run television said a commission would be formed to investigate the incident.
Myanmar, a predominantly Buddhist nation of 60 million, emerged from a half-century of military rule in 2011. But newfound freedoms of expression that accompanied its transition to democracy have given voice to religious hatred, causing violence that has left up to 280 people dead and sent another 140,000 fleeing their homes.
Most of the victims have been members of the long-persecuted Rohingya Muslim minority.
Aid groups that have been providing care for those now living in crowded camps - where they have little access to food, education or health care - have for months faced threats and intimidation by Buddhist Rakhine, hampering their ability to work.
Last month, the government stopped the Nobel Peace Prize-winning aid group, Doctors Without Borders, from working in the state altogether, in part because it had hired Rohingya.
Tensions in Rakhine have reached fever pitch ahead of next month's national census - the first in 30 years. Many Buddhists say members of the religious minority should not be allowed to identify themselves as Rohingya - while not listed among the 135 ethnicities, there is an "other" category where respondents could write it in - over fears it could legitimise their existence in the country.
Though many of their families arrived generations ago, they have been denied citizenship by law.
As part of the anti-Rohingya campaign, Buddhist flags have been place in front of almost every house and office in Sittwe in recent days.
Up to 300 people surrounded Malteser International late Wednesday following reports that a woman had removed the flag from the group's office, Rakhine state spokesman Win Myaing said, adding that police had to fire 40 to 50 warning shots to disperse the crowd.
The organisation could not immediately be reached for comment, but residents said the woman who took down the flag was seen holding it near her waist, a sign of disrespect.
The violence continued Thursday, with more than 1000 people running through a street that houses international aid workers, throwing rocks at homes and damaging several of the residences.
"If police stopped them at one place, the mob moved to a different location and threw stones at (non-governmental organisation) houses," Sittwe resident Aung Than said by phone.
Police escorted aid workers from their homes for safety reasons on Thursday, he said.
Dozens were taken to a guest house.
Other aid groups said they were evacuating all local and foreign non-essential staff from Sittwe, some on regularly scheduled flights, others on charters. Nearly a dozen arrived in Myanmar's main city of Yangon on Thursday afternoon, some carrying blue "Save the Children" bags.
Among those shepherded to safety on Thursday were three Americans.
The US Embassy in Yangon said it was deeply concerned by the mob violence and the inability of security forces to stop its spread.
"Unhindered and regular humanitarian access to communities in need, and the safety and security of humanitarian aid workers to ensure the effective delivery of these services, are core international principles," it reminded the government in a statement.
Local Rakhine residents have been angry with international non-governmental staff since communal violence first erupted in mid-2012, accusing them of being biased in favor of the Muslim community. There have been several peaceful protests in the past, but this is the first time property of the international aid organizations has been so directly targeted.
Authorities were driving around the city announcing the 6 p.m. to 6 a.m. curfew through loudspeakers, said Tun Tha, a Sittwe resident, adding that soldiers and police were being stationed near the offices of the United Nations and international aid groups.
A statement by the UN resident and humanitarian coordinator in Myanmar, Toily Kurbanov, urged the government to protect aid workers and warned that "any reduction of humanitarian presence could negatively affect the protection of vulnerable people."
He called on authorities to make sure those responsible would be held accountable.
Almost all of Myanmar's 1.3 million Rohingya live in Rakhine. Some descend from families that have been here for generations. Others arrived more recently from neighboring Bangladesh. All have been denied citizenship, rendering them stateless. For decades, they have been unable to travel freely, practice their religion, or work as teachers or doctors. They need special approval to marry and are the only people in the country barred from having more than two children.