"We have a tour group setting off on Thursday, but so far we have not received any notice from the Shanghai Municipal Tourism Administrative Commission. According to our previous experience, the authorities usually release the notice on the second day," said a sales manager, surnamed Gu, at Qiangsheng Travel, adding that the trips to Bangkok have not been affected.

A tsunami watch means there is the potential for a tsunami, not that one is imminent.

President Susilo Bambang Yudhoyono told reporters: "There is no tsunami reported so far, but we remain vigilant."

"Our warning system is working well, and I have ordered the national relief team to fly immediately to Aceh to ensure the situation is under control and to take any necessary action," he said.

"Thank God, from what I heard there are neither casualties reported nor major damage in Banda Aceh or other places," he added.

There was chaos in the streets, with fierce shaking continuing for nearly four minutes.

Patients in several cities poured out of hospitals, some with drips still attached to their arms. In some places, electricity was briefly cut.

"It wasn't the strongest quake I've felt," said 22-year-old Tuti Rahmi, while trying to reach her brother by phone.

"But it seemed to last forever," she said. "Hopefully there won't be too much damage."

One hour after the temblor, people were still standing outside their homes and offices, afraid to go back inside.

There were several strong aftershocks.

"I was in the shower on the fifth floor of my hotel," Timbang Pangaribuan told El Shinta radio from the city of Medan. "We all ran out ... We're all standing outside now."

He said one guest was injured when he jumped from the window of his room.

Thailand's National Disaster Warning Center issued an evacuation order to residents in six provinces along the country's west coast, including the popular tourist destinations of Phuket, Krabi and Phang-Nga.

AP-China Daily

A scientist points at a screen at the German Research Centre for Geosciences in Potsdam that shows the epicenters of an earthquake and its aftershocks off the coast of Indonesia, April 11, 2012. [Photo/Agencies]