Relatives of a victim hug as they wait at a hospital where injured people of a stampede incident are treated in Shanghai on Thursday. Aly Song / Reuters

At least 35 people were killed and more than 40 injured in a stampede during New Year's Eve celebrations in Shanghai, possibly caused by people rushing to pick up fake money thrown from a building overlooking the Bund waterfront district.

The Shanghai government said that large crowds started to stampede in Chen Yi Square on the Bund just before midnight.

The cause of the stampede has not been officially confirmed, but a witness said the chaotic scene started when people attempted to pick up coupons made to look like dollars thrown from a building.

A man who took one of 43 people injured to a local hospital for treatment said the fake dollar notes had been thrown from a bar above the street, and people rushed to pick up the money. The Paper of Shanghai reported that the coupons were thrown from the 18th floor of a high-rise building.

Wu Tao, who helped rescue an injured woman, told The Paper that the mad scramble continued for about three minutes.

"There were just too many people and nowhere people could escape to," said a woman who declined to give her name.

"We were down the stairs and wanted to move up, and those who were upstairs wanted to move down, so we were pushed down by the people coming from upstairs," an injured man told Shanghai TV. "All those trying to move up fell down on the stairs."

"Some people have fallen," Shanghai police soon warned on Weibo, a Twitter-like service, and they urged people to obey police and leave the scene without pushing.

Weibo photos early had shown densely packed crowds of revelers along the Bund, where buildings from Shanghai's earlier days face the Huangpu River.

One photo shared by official news agency Xinhua showed at least one person doing chest compressions on a shirtless man while several other people lay on the ground amid debris. Another photo showed the area ringed by police. The Shanghai city government released photos online showing the mayor hurrying into a local hospital to visit victims.

Dozens of distraught relatives gathered in the hospital lobby waiting for news, with some expressing frustration over a lack of information as police restrained them.

"Many relatives have asked to go inside and asked the hospital to give us a list of the injured, including the conscious and unconscious ones who are being treated in there, but nobody got back to us," said a relative who gave her family name as Fan. "Six hours have passed, but we still don't have any information," she added.

CCTV America posted video showing piles of discarded shoes amid the debris.

Xinhua reported that many of the injured were students.

President Xi Jinping has told the Shanghai government to get to the bottom of the incident as soon as possible, and ordered governments across the country to ensure a similar disaster never happens again, state television said.

The Shanghai government said on its official microblog that an inquiry had begun.

Shanghai Daily reported last week that the annual New Year's Eve countdown on the Bund that normally attracts about 300,000 people had been canceled, apparently because of crowd control issues.

The report said a "toned-down" version of the event would be held instead but that it would not be open to the public. The Bund runs along an area of narrow streets amid restored old buildings, shops and tourist attractions in China's largest urban area.

At dawn on Thursday, there were still small crowds of revelers trying to find taxis home as workers cleared trash.

In 2004, 37 people died in a stampede in northern Beijing, on a bridge at a scenic spot, during the Lunar New Year holiday.