The stoppage -- part of rolling industrial action the union threatened to continue until Friday -- was due to hit Frankfurt, Duesseldorf and Munich airports tomorrow, the UFO flight attendants' union said.
Lufthansa said it would cancel 929 of the day's 3,000 scheduled flights to or from the three cities, affecting 113,000 passengers, but that about 70 percent of its normal services would operate.
The airline also said its executive board would tomorrow discuss the "consequences" of the strike, which it called "unprecedented in the history of Lufthansa", and issue a statement to employees and the public around 1700 GMT.
The carrier voiced regret about the strike, apologised to customers and said it would publish new flight plans on www.Lh.Com and inform passengers of the status of their bookings by email and SMS.
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Frankfurt, Germany's main air hub, and Duesseldorf were to be hit by work stoppages from 0330-2200 GMT, and Munich from 0330-2300 GMT, the union said on its website on today.
UFO last Thursday said industrial action had become "unavoidable" after airline management failed to come up with an improved offer in a dispute over pay and early retirement provisions.
The strike yesterday forced Lufthansa to cancel 520 flights, leaving 58,000 passengers grounded. The stoppage affected all domestic and European flights from Frankfurt and Duesseldorf.
Flights resumed for one day today "because on that day, most flights are private", said UFO.
UFO has said it plans to target different airports over the course of the week-long blitz of walkouts.
The union is demanding that a current system of early retirement provisions remain unchanged.
Lufthansa has argued that the system is too expensive in the face of competition from low-cost operators such as Ryanair and easyJet.