At least 31 people have been killed and seven others injured in a massive explosion at a restaurant in northwestern China, authorities said Thursday.

The explosion was caused by cooking gas at a barbecue restaurant on the eve of a long holiday weekend.

The blast tore through the restaurant at around 8:40 p.m. Wednesday on a busy street in Yinchuan, the capital of the traditionally Muslim Ningxia Hui Autonomous Region, as people gathered ahead of the Dragon Boat Festival, the official Xinhua News Agency said.

The explosion left many people unconscious and they needed to be carried out of the shop, according to the online news site The Paper, which quoted a member of the search and rescue team. Victims included elderly people and high school students, it added.

An hour before the explosion, employees noticed the smell of cooking gas and discovered that a gas tank valve was broken, according to Xinhua. The blast occurred while an employee was replacing the valve.

Chinese President Xi Jinping demanded urgent medical care for the injured and a safety overhaul after the explosion, Xinhua said. He urged efforts to quickly determine the cause of the accident and hold people accountable under the law.

Xi also said all regions and related departments should address "all types of risks and hidden dangers" and launch campaigns to promote workplace safety.

Nine people have been detained by authorities, including the owner of the restaurant, shareholders and employees. Their assets have also been frozen, according to Xinhua.

The blast happened at an outlet of Fuyang Barbecue, a chain restaurant in Yinchuan popular for its grilled skewers and stir-fried dishes, The Paper said.

The two-floor restaurant could seat 20 people on the ground floor and offered private dining rooms on the second floor where customers could also sing karaoke.

The Paper cited a woman identified only by her surname Chen saying she had been about 50 meters (164 feet) from the restaurant when she heard the explosion.

She described seeing two waiters emerge from the restaurant afterward, one of whom collapsed immediately, while thick smoke billowed from the restaurant and a strong smell of cooking gas permeated the area.

The central government's Ministry of Emergency Management said on social media that search and rescue work at the restaurant was completed early Thursday morning and investigators were working to determine the cause of the blast.

The Dragon Boat Festival is a holiday devoted to eating rice dumplings and racing boats manned by teams of paddlers. While the majority of Yinchuan’s population is Han Chinese, a third are Hui people, or Chinese Muslims.

Industrial accidents of this type are a regular occurrence in China, usually attributed to poor government supervision, corruption, cost-cutting measures by employers and little safety training for employees.

At least nine people were killed in an explosion at a Chinese petrochemical plant, and three others died in a helicopter crash during the country’s May Day holiday.

In February, 53 miners were killed in the collapse of a massive open pit coal mine in the northern region of Inner Mongolia, leading to numerous arrests, and four people were detained over a fire at an industrial trading company in central China in November that killed 38 people.

The central government has pledged stronger safety measures since an explosion in 2015 at a chemical warehouse in the northern port city of Tianjin killed 173 people, most of them firefighters and police officers. In that case, a number of local officials were accused of having taken bribes to ignore safety violations.