A seven-block perimeter of downtown Montreal remained cordoned off Saturday and part of a subway line remained closed as workers installed a series of posts to reinforce a cracked tunnel.

Métro changes and alternate routes:

The risk of collapse forced authorities to close part of the Métro service, at least for the weekend.

STM says there will be no service on the Green line between the Berri-UQAM and Lionel-Groulx stations for the weekend.

The transit system will offer temporary bus service between these stations:

  • Eastbound, on Sainte-Catherine Street.
  • Westbound, on René-Lévesque Boulevard.

The transit service also suggested riders could take the Orange line between Berri-UQAM and Lionel-Groulx stations.

Problems were firstspotted around 1:30 p.m. ET Friday, when employees working in the basement of the landmark Bay department store on Maisonneuve Boulevard noticed water leaking from the ceiling, which haddropped a few centimetres.

Authorities emptied buildings and sealed off a large section of the downtown core, as well as cancellingservice on part of one subway line after the fissure and a second onewere discovered inthe tunnel linking the McGill Métro station to malls.

On Saturday, city officials told reporters that about 1,000 posts were being erected to support a slab of concrete, which has two cracks extending the length of a city block.

Montreal Mayor Gérald Tremblay said the security perimeter would remain and no cars would be allowed to enter the area until authorities feel certain the 40-year-old structure is safe.

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Engineers and city employees survey the streets around The Bay department store in Montreal, Saturday. (CP)

Several hundred people were forced to leave the Bay and the McGill station was shut down as a precaution, causing commuter and traffic hassles.

The tunnel, running between Aylmer Street and Union Avenue, was at risk of collapsing, officials said.

The Métro's Green line between Berri-UQAM and Lionel-Groulx stations will be closed at least until Monday, transit officials say. They're encouraging people to use the Orange line.