Tight new security regulations ahead of a politically sensitive event in China's troubled western region of Xinjiang have sparked travel chaos around the country as airports are ordered to carry out extra security checks.
Beijing is ramping up security ahead of the China-Eurasia Expo which opens Thursday in Urumqi, where deadly ethnic riots killed at least 200 people two years ago.
Two days ahead of the trade expo, official media said the convention center in suburban Urumqi was guarded by police carrying rifles, while S.W.A.T. units were deployed to handle the security check at the entrance.
Officials have also declared a "no-fly zone" over the city, banning "anything from light aircraft to hot-air balloons," the state-run Xinhua news agency reported.
"A unit of the Snow Leopard Commandos, an elite counter-terrorism force under the People's Armed Police, has been deployed from Beijing to the region," the agency said.
Meanwhile, new security rules at major airports in 12 Chinese cities tightened security screenings for Urumqi-bound flights from Sunday, with passengers asked to remove their shoes and have their carry-on luggage open for inspection.
An employee who answered the phone at the Beijing Capital Airport help desk confirmed the new security measures.
"Currently, passengers on all flights are required to submit to stricter security checks," she said. "We suggest that you arrive at the airport three hours ahead of your flight time so as to make your flight."
But she declined to comment on the reason for the heightened security, which online rumors suggested was linked to the discovery of a hidden knife in a water bottle at a Xinjiang airport in recent days.
"I don't know the actual situation," she said. "We haven't received any orders from higher authorities yet."
Around 70 domestic flights were delayed as the new rules came into force, with many passengers missing flights because of the extensive security measures, officials media reported.
Photos circulated online of huge crowds in departure halls at Beijing, Shanghai, and Guangzhou airports, with travelers airing their frustrations via popular microblogging services like Sina Weibo.
"Waiting to clear security at Guangzhou airport, which is packed with people," wrote Sina user @laofengjiadesanbao on Monday. "The security checking process is very slow."
The user added: "There is a special queue for flights heading to Urumqi. The atmosphere is a bit tense."
Meanwhile, in Shanghai, passengers were being warned via official media websites to get ready for additional security checks.
"In order to comply with Air China security regulations, passengers are now required to take off their shoes and their belts," the city's Dongfang news website reported.
Chinese authorities in the troubled western region of Xinjiang have recruited thousands of new security personnel, beefing up guard on streets, apartment blocks, and shops following a string of violent incidents in the region.
Urumqi residents said the new recruits have been placed on duty following the announcement of a "strike hard" campaign targeting "terrorism" and crime earlier this month.
Hu Jun, an online rights activist who lives in Changji county on the outskirts of the city, said checkpoints had gone into full force at the weekend.
"All the vehicles are being subjected to full identity checks, or they won't be allowed to enter the city," Hu said on Monday. "This started the day before yesterday."
An Urumqi resident surnamed Wang said the authorities had stuck posters up around the city warning of additional security checks in Urumqi and neighboring Changji, Bazhou, and Yelupan districts.
"Nothing is being allowed to fly over the city, including pigeons, helium balloons, kites, and Chinese lanterns," Wang said.
A second Urumqi resident who declined to be named said that families living on streets near the expo venue were being ordered not to open their windows.
"They have been informed across that whole area that they mustn't open their windows," the resident said. "This is effective from the end of August until the expo ends."
Hu said the authorities appeared to want to create a tense atmosphere.
"Everything is under military control," he said. "Every day we hear propaganda broadcasts warning that they are doing door-to-door checks to search for out-of-towners and check their papers."
"Things have been pretty tense here these past few days."
Hu said he believes the security regulations have a longer-term goal behind them.
"If you put everything on a war footing, you can centralize and strengthen control," he said. "I think the government wants to create a tense atmosphere to show off its military strength ... There is no concrete reason for it."
This year's Islamic holy month of Ramadan, which runs through most of August, has brought a fresh clampdown on Muslims in the wake of a July 31 attack in the Silk Road city of Kashgar that left at least 14 people dead and 40 others injured.
The attacks in Kashgar and in the nearby city of Hotan two weeks earlier, in which 20 people were killed, mark the bloodiest violence in a year in Xinjiang, where many Muslim Uyghurs chafe under Chinese rule.
Reported by Ding Xiao and Qiao Long for RFA's Mandarin service, and by Fung Yat-yiu for the Cantonese service. Translated and written in English by Luisetta Mudie.