A major port linking Hong Kong and Shenzhen resumed operations on Tuesday morning after firefighters put out a blaze in a customs building on Monday night.

At 6:30 am Tuesday, Luohu port in Shenzhen re-opened to passengers traveling between Shenzhen and Hong Kong.

The customs building caught fire on Monday, stranding thousands of passengers.

According to the Shenzhen Port Office, the fire began at 7: 40 pm. Port staff were unable to put out the fire with fire extinguishers and at 8 pm called police.

The Shenzhen fire department dispatched 6 fire engines and 33 firemen to the blaze. Firefighters saved 20 people trapped on the top floor of the building.

The blaze was put out at 8:30 pm. There were no injuries reported or loss of property.

Fire investigators said the fire could have been caused by a short circuit on the third floor of the building, which threatened the main electrical room on the fifth floor.

Passengers were re-routed through Huanggang Port and Futian Port, which extended its hours by one-and-a-half hours to 12 mid-night.

A Hong Kong freight agent who travels between the two cities several times a day said he was in Sheung Shui in Hong Kong at the time the fire broke out.

"I could see the smoke from Hong Kong, and it was very heavy," said the agent who gave his surname as Li. "Later I heard from my friend who was waiting to pass through customs that there was a fire on the third floor of the customs building, where the Hong Kongers pass."

"It was peak time for customs, there were at least 7,000 or 8,000 people stranded in Luohu Port," he continued. "Around 7 or 8 pm, officers began to evacuate people to other ports like Futian Port and Huanggang Port. I got here from Shenzhen Bay Port."

More than 100, 000 passengers travel daily through Luohu Port between Shenzhen and Hong Kong.