OTTAWA, Canada -- Relentless rain battered Canada's Pacific coast on Monday, forcing a town's evacuation and trapping motorists as mudslides, rocks and debris were washed across major highways.

Some 275 people, according to local media, were stuck overnight in their cars between two mudslides on Highway 7 near the town of Agassiz in British Columbia.

Since the morning, additional mudslides near Lillooet and Haig pinned down more travellers, the province's public safety minister, Mike Farnworth, told a news conference.

Meanwhile, Merritt -- about 300 kilometres (185 miles) from the coast -- ordered the evacuation of all 7,000 of its townsfolk after flooding compromised the local wastewater treatment plant and washed out two bridges. Barricades also went up restricting access to the town.

Farnworth said search and rescue crews were dispatched to free people trapped for hours without food or water in 80 to 100 cars on Highway 7.

"Many people have been rescued by helicopters from mudslides near Agassiz and Hope with crews working to rescue the remaining people in the next few hours," he said.

Those trapped in 50 vehicles in the Lillooet mudslide have all been rescued, while efforts were underway to free an unspecified number of people at the Haig site, he added.

Video footage showed a military helicopter landing on a highway covered in mud and debris, to pick up stranded motorists.

British Columbia emergency health services said it transported nine patients to hospital with minor injuries overnight from the Agassiz landslide.

Emergency centres were set up for displaced residents.

"Please stay safe," Prime Minister Justin Trudeau said in a Twitter message to British Columbians.

"We're ready to provide whatever assistance is needed as you deal with and recover from the flooding and this extreme weather," he said.

- Wettest ever -

British Columbia's transportation ministry said several highways were closed Monday. "Heavy rains and subsequent mudslides/flooding have impacted various highways in the BC interior," it said.

The local utility issued flood alerts due to high water flows into its reservoirs, and said it was working to restore power to thousands hit by outages.

Construction of the Trans Mountain pipeline connecting the Alberta oil sands to the Pacific coast was also paused, a company spokesperson told AFP, "due to widespread flooding and debris flows."

In the city of Abbotsford, outside Vancouver, authorities ordered more than 100 homes evacuated in several neighborhoods threatened by flooding and mudslides, while television images showed farms in the Fraser Valley under several feet of water.

Meteorologist Tyler Hamilton commented on social media that Abbotsford in the past 140 days had experienced both its warmest and wettest days ever.

Environment Canada said up to 250 millimetres (almost 10 inches) of rain -- what the region normally gets in a month -- was expected by the day's end in and around Vancouver, which was also hit last week by a rare tornado.

The extreme weather comes after British Columbia suffered record-high temperatures over the summer that killed more than 500 people, as well as wildfires that destroyed a town.