China's transport system was tested on Wednesday as tourists returning from their May Day holiday trips pushed traffic flow in many places to record highs.

There were an estimated 2 million vehicles on the highways in Beijing on Wednesday, the last of the three-day holiday, a 41 percent rise from last year, according to the Beijing Municipal Commission of Transport.

A nationwide toll-free policy was implemented during the holiday, starting from Monday. It boosted tourism and increased the number of vehicles on the road.

Passengers enter the railway station in Yinchuan, the Ningxia Hui autonomous region, on Wednesday, the last day of the May Day holiday. Photo by Peng Zhaozhi / Xinhua

The Beijing traffic authority said on Tuesday that the traffic congestion index reached 9.5 just before noon within Fifth Ring Road, the main urban area. The reading, on a scale from zero to 10, means "heavily congested".

From Wednesday afternoon, returning vehicles caused congestion along the sections of the Beijing-Tibet, Beijing-Chengde and Beijing-Kaifeng highways.

Traffic authorities said traffic jams in Beijing's main urban area will continue until Sunday because of the short interval between the May Day holiday and the weekend.

According to a statement from the Ministry of Transport, the highway network saw 21 major cases of congestion across the country as of 4 pm on Wednesday, including six heavy congestion cases caused by accidents.

The most heavy traffic flow appeared in the Shanghai section of the Shenyang-Haikou Highway, the Beijing section of the Beijing-Shanghai Highway and the Zhejiang section of the Shanghai-Kunming Highway, the statement said.

Railways also witnessed heavy pressure. On Monday, 8.88 million rail journeys were made in China, a record high for any May Day holiday.