Thousands of London Underground workers went on strike Sunday after their unions rejected a last-minute offer to avert a 24-hour walk out, putting the U.K. capital on course for transportation chaos during Monday morning’s commute.

Staff at stations in London’s 150-year-old underground rail network, known as the Tube, began leaving their posts at 6 p.m., forcing Transport for London (TFL) to shut service and warn of disruptions. Riders on the system, which on peak days can carry as many as 4.8 million passengers, were told to allow extra time to travel.

“This tube strike will cause misery to millions of Londoners,” London Mayor Sadiq Khan tweeted just after 6 p.m., when the walkout was set to begin.

Services were to be “severely reduced” on Sunday and Monday, the transit operator said in an e-mailed statement 90 minutes before the action began. Additional buses will be added to the schedule but buses, roads and rail services are expected to be much busier than usual, it said.

The strike is the first of 2017 after walk outs last year over job cuts and ticket-office closures initiated by then Mayor Boris Johnson, paralyzing the city and leaving commuters scrambling for alternative ways to work. The walkout by Underground workers coincides with planned strikes this week at Southern Railway, which provides commuter services across most of southeast England.

More than 3,000 members of the Rail, Maritime and Transport union, known as the RMT, voted on the strike, with 85 percent backing action after 900 layoffs and closures hurt safely, according to a statement on RMT’s website that cited a terror alert at North Greenwich last year and an incident at Canning Town when a passenger fell between a train and a platform.

“Our reps have decided to go ahead with our 24 hour strike action,” Manuel Cortes, general secretary for Transport Salaried Staffs’ Association, or TSSA, the other union representing Tube workers, said in a statement posted on the organization’s website.

Before the walkout began, Cortes said the TFL’s offer to increase staff was “a step in right direction” but fell short of the “urgency that is now needed.”

Passengers should allow extra time to travel and avoid peak hours as Tube services are “severely reduced” and some stations are closed, Transport for London said on its website. Other forms of transport such as buses and trains running above ground will be “busier than usual.” Full service is expected to resume by Tuesday morning, TFL said.

Reduced services will operate on the District, Circle and Hammersmith & City lines—which operate mostly in central London—although trains will not stop at all stations, according to TFL’s website. The Victoria and Waterloo and City lines will be shut and Piccadilly Line trains will stop at Heathrow terminals 1-3, not terminals 4 and 5, TFL said.