Visitors attending Europe’s biggest travel fair were stranded in Berlin as striking ground workers forced almost all flights in and out of the German capital to be canceled.

About 670 arrivals and departures were scrapped at Berlin’s Tegel and Schoenefeld airports on Friday, after the Ver.di union called on more than 2,000 ground workers to walk out in a pay dispute. The one-day action comes just as the ITB Berlin fair, which draws 180,000 visitors annually, gears up for its busiest period.

Workers are seeking higher pay following five rounds of unsuccessful negotiations, as European Union rules to bring more competition to ground handling have squeezed wages. Ver.di has already held warning strikes this year and more disruptions are possible in coming weeks. The dispute adds to Berlin’s aviation embarrassments, with the opening of its new airport delayed until 2018 following a series of technical fiascos and budget overruns.

The top carriers at Tegel airport are Air Berlin Plc, Deutsche Lufthansa AG and British Airways. The biggest ones serving Schoenefeld are Ryanair Holdings Plc, EasyJet Plc and Norwegian Air Shuttle ASA.

Lufthansa scrapped its entire flight program to and from Berlin on Friday, resulting in 68 cancellations. Air Berlin warned its customers on Thursday to expect “massive’’ travel disruptions, and said they can rebook their tickets, trade them in for rail vouchers within Germany, or request refunds. Valuables should be taken in carry-on luggage only, as baggage handling will also be affected, the airline said.

Wage Dispute

Workers are demanding a raise of 1 euro per hour under a one-year contract, representing an increase of more than 10 percent for the lowest wage group which earns 9.30 euros ($9.86) per hour. Employers, which include Wisag, Aviation Solution and Aeroground Berlin, offered an 8 percent raise over three years, Ver.di said.

The union is also negotiating pay for ground-handling employees in Frankfurt, Dusseldorf, Cologne, Leipzig, Dresden and Stuttgart, where 300 workers held a warning strike earlier this month. The union reached an agreement in Hamburg in February that would give ground workers about 200 euros more per month over two years, though members have yet to accept the settlement.

Berlin’s airports served a combined 32.9 million passengers in 2016, a gain of 11 percent from a year earlier.

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Photo Credit: Ground workers at Berlin's two major airports have gone on strike. In this January. 20, 2013 photo, an Air Berlin plane lifts off at Tegel airport in Berlin. Michael Sohn / Associated Press