A multimillion-dollar bridge on the Trans-Canada Highway, the sole east-west route across part of northern Ontario, has partially reopened after sustaining serious damage over the weekend, provincial officials said Monday.

The Ontario Provincial Police and the Ministry of Transportation confirmed that one lane of the Nipigon River Bridge has reopened.

A statement from Transportation Minister Steven Del Duca said the lane is available to cars and regular-weight transport trucks, but that engineers are still working to determine whether it can sustain the weight of oversized trucks.

One lane of the Nipigon River Bridge on the Trans-Canada Highway has reopened. (Martine Laberge/Radio-Canada)

"As soon as I heard about the closure of the bridge I was concerned, because it is the lifeline in terms of product and transportation in northwestern Ontario," Premier Kathleen Wynne said.

"It connects the east to the west, and there is only that one route."

There were no injuries reported, and pedestrians were still able to cross the bridge.

Northern Development and Mines Minister Michael Gravelle will get a first-hand look at the Nipigon River Bridge on Wednesday and get an update from engineers who are assessing the extent of the damage and how long it will take to repair. (Facebook)

Word of the partial reopening came as a relief to local businesses, said Dan Bevilacqua, manager of the North of Superior Travel Association.

He said companies were bracing to function without bridge access for anywhere from two days to a month, adding that the flow of supplies wasn't even the main concern.

"I think what people were worried about is just essentially the country being split in two," he said. "Here in Nipigon, we are at the middle of the country. Everything has to come through this area.… It was more concern for travellers."

No one has yet offered an estimate as to when the bridge, which spans the Nipigon River, will be fully functional.

Nor is there any word as to what caused the separation, an issue Wynne said the government is determined to "get to the bottom of."

Trucks starting to roll eastbound out of <a href="https://twitter.com/hashtag/Nipigon?src=hash">#Nipigon</a>. <a href="https://twitter.com/hashtag/cbctb?src=hash">#cbctb</a> <a href="https://twitter.com/hashtag/tbay?src=hash">#tbay</a> <a href="https://t.co/YKOgtZzRix">pic.twitter.com/YKOgtZzRix</a> —@JeffWaltersCBC

On Sunday, Mayor Richard Harvey said the only option for motorists crossing northern Ontario was to detour through the U.S. It is the only road in northern Ontario that connects Eastern and Western Canada.

Harvey told CBC News the bridge closure is part of his community's emergency plan.

"If this is something that is going to take a little longer, we do have other options that we'll be looking at to get traffic moving as quickly as possible," he said.

Currently the township is opening up community centres to give anyone stranded a place to stay.

Provincial police had been stopping people heading out on the Trans-Canada Highway at larger Ontario centres such as Sault Ste. Marie, Thunder Bay and Terrace Bay, telling them to turn back or use an alternate route through the U.S.

Crews worked late Sunday night on the Nipigon River Bridge, with the Ministry of Transportation pledging to safely restore the bridge as soon as possible. (Jeff Walters/CBC)

Ontario Provincial Police said part of the steel decking is sticking up about 60 centimetres, and images of the damage show it spans the entire width of the vehicle lanes for the bridge.

The Municipality of Greenstone, which includes a number of small communities in the area, declared a state of emergency Sunday because of the bridge closure.

Pickup trucks 'smashed their front ends'

A resident in nearby Dorion, Ont., witnessed the failure Sunday afternoon as she was driving with her husband.

"As we turned [onto the highway], we saw the whole bridge — a kind of big gust of wind came underneath it and blew it up and then it came back down," Ashley Littlefield said, adding it shifted by about half a metre.

"We watched two pickup trucks come flying over.… They didn't see us, didn't hear my horn honking, and they flew over and smashed their front ends down on the cement."

Littlefield said her husband got out of the vehicle to stop traffic, before police arrived about 10 minutes later.

One lane of traffic open after temporary closure 1:28

The Ontario government began building what it touts as the province's first cable-stayed bridge in 2013 and opened westbound lanes to two-way traffic in November. The project, which the government pegs at $106 million, is due to be completed in 2017.

Ontario Northern Development and Mines Minister Michael Gravelle was expected to visit the site on Monday to assess the extent of the damage, and Del Duca is planning to inspect the site on Wednesday.

Ontario's New Democrats said the failure of the bridge "shows the Liberal government's mismanagement of northern Ontario's roads and highways."

"Many companies in northern Ontario depend on the bridge to transport product across the country," said Wayne Gates, the party's transportation critic. "This will hurt industry, and many small communities will be economically impacted with less motorists passing through."