Chinese rescue workers were still digging through rubble Thursday in a desperate hunt for survivors following a series of explosions the day before at a hazardous chemicals warehouse in the port city of Tianjin that killed at least 50 and injured more than 700.

The municipal government in Tianjin, a key city about 75 miles east of Beijing, said 701 people were injured, including 71 in serious condition. It gave no figure for the missing

More than 1,000 firefighters had been sent to the city to battle fires that had been set off by the blasts, which began late Wednesday at the warehouse and caused huge, fiery blasts that knocked doors off buildings in the area, shattering windows several miles away and turned nearby buildings into skeletal shells.

"I thought it was an earthquake, so I rushed downstairs without my shoes on," Tianjin resident Zhang Siyu, whose home is several kilometers from the blast site, said in a telephone interview. "Only once I was outside did I realize it was an explosion. There was the huge fireball in the sky with thick clouds. Everybody could see it."

Zhang said she could see wounded people weeping. She said she did not see anyone who had been killed, but "I could feel death."

More On This...

There was no indication of what caused the blasts, and no immediate sign of any large release of toxic chemicals into the air. The Beijing News newspaper reported on its website that there was some unidentified yellow foam on the ground at the site, although it was not clear if this was contamination or part of the fire-fighting efforts.

Police in Tianjin said an initial blast took place at shipping containers in a warehouse for hazardous materials owned by Ruihai Logistics, a company that says it is properly approved to handle such materials. State media said senior management of the company had been detained by authorities, and that President Xi Jinping has demanded severe punishment for anyone found responsible for the explosions.

Ruihai Logistics says on its website that it was established in 2011 and is an approved company for handling hazardous materials. It says it handles 1 million tons of cargo annually.

The official Xinhua News agency said an initial explosion triggered other blasts at nearby businesses. The National Earthquake Bureau reported two major blasts before midnight, the first with an equivalent of 3 tons of TNT, and the second with the equivalent of 21 tons.

"It was like what we were told a nuclear bomb would be like," said truck driver Zhao Zhencheng, who spent the night in the cab of his truck. "I've never even thought I'd see such a thing. It was terrifying, but also beautiful."

The explosions took place in a mostly industrial economic development zone, with some apartment buildings in the vicinity. Buildings of a half-dozen other logistics companies were destroyed in the blasts, and more than 1,000 new cars were left charred in a nearby parking lot, the Beijing News said.

As is customary during disasters, Chinese authorities tried to keep a tight control over information. Police kept journalists and bystanders away with a cordon a few miles from the site. On China's popular microblogging platform of Weibo, some users complained that their posts about the blasts were deleted, and the number of searchable posts on the disaster fluctuated, in a sign that authorities were manipulating or placing limits on the number of posts.

Photos taken by bystanders and circulating on microblogs show a gigantic fireball high in the sky, with a mushroom-cloud. Other photos on state media outlets showed a sea of fire that painted the night sky bright orange, with tall plumes of smoke.

About 1.2 miles from the explosion site is the luxury Fifth Avenue apartment complex on a road strewn with broken glass and pieces of charred metal thrown from explosion. Like surrounding buildings, the Mediterranean style complex had all its windows blown out, and some of its surfaces were scorched.

"It's lucky no one had moved in," said a worker on the site, Liu Junwei, 29. "But for us it's a total loss. Two years of hard work down the drain."

"It had been all quiet, then the sky just lit up brighter than day and it looked like a fireworks show," said another worker on the site who gave just his surname, Li.

Tianjin, with a population of about 15 million, is about 75 miles east of Beijing on the Bohai Sea and is one of the country's major ports. It is one of China's more modern cities and is connected to the capital by a high speed rail line.

The Associated Press contributed to this report.

Click for more from Sky News.