The stoppage will target passenger flights across Germany from Monday at 1pm until midnight on Tuesday, the union said in a statement on Sunday.
Pilots' union Cockpit said on Monday morning that long-haul pilots would also strike from 6am on Tuesday until midnight. So far 120 flights have been cancelled at Berlin's Tegel Airport.
In Hamburg, more than 30 flights have been cancelled. You can see a list of cancelled flights here.
Lufthansa said that it was surprised by the extension of the strike to long-haul flights, but said it would press on with a plan to reveal an emergency timetable by midday on Monday.
The announcement came just hours before train operator Deutsche Bahn announced that service was returning to normal on Monday morning after a 50-hour strike by drivers ended at 4 am, although there were still a few delays and cancellations.
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“The Vereinigung Cockpit (VC) announces the start of further strikes for Monday, October 20th,” the statement said.
“Regrettably Lufthansa has not acted on the compromise proposals of VC after seven strikes now since April this year and is stonewalling,” it added.
The union said it regretted the disruptions for passengers and called on Lufthansa to give up its “blockade attitude”.
Lufthansa said late on Sunday that the strike would affect some 2,150 flights and more than 200,000 passengers, but that it had found enough willing pilots to ensure “a third” of the flights would be running.
The airline accused the union of trying to turn Germany into a “nation at standstill” with the eighth call to stop work in less than two months.
A successful economy cannot afford such strikes, Lufthansa said in a statement.
Lufthansa pilots can currently take paid early retirement from the age of 55. They are fighting a plan by the airline to raise the minimum age and to involve pilots in the financing of their pensions.
Last week, pilots held a 12-hour strike at Germanwings, the low-cost airline belonging to Lufthansa.