Stores and subway service in Montreal's downtown commercial core are open again after crews secured a sagging concrete slab in an underground tunnel over the weekend.

A tunnel linking the Bay department store to the city's subway network was deemed at risk of collapse Friday because cracks were found in the slab, which had dropped less than 2.5 centimetres.

Crews worked through the weekend to shore up the tunnel with about 1,000 posts placed in a criss-cross structure.

"I was rather reluctant to take it (the train) because of the cracks,'' Pat Bucchianico, who said his train was delayed apparently by a man walking on the tracks, told The Canadian Press.

"Other than that it was rather OK.''

Above ground, business owners in the commercial district that was shut down were anxious to reopen.

"I lost two days,'' said Meng Meng, a magazine store owner. "After two o'clock Friday afternoon there weren't many people.''

The affected area was bordered by Sherbrooke, Ste. Catherine and Bleury Streets, as well as University Avenue, and is home to Montreal's Eaton Centre as well as hundreds of stores and restaurants.

"These cracks look like they're fairly new, they're sheer cracks," engineer Hellen Christodoulou told CTV Montreal on Monday. "I think the problem is probably much more serious."

Mayor Gerald Tremblay said Sunday that it took "remarkable teamwork'' over the last three days to ensure the area was secure.

The block of de Maisonneuve Boulevard where the cracks were found will remain closed to vehicles but will be open to pedestrians, Tremblay said.

Although he declared the area safe, Tremblay said permanent repairs are still needed.

With a report from CTV Montreal's Paul Karwatsky and files from the Canadian Press