SAS said 673 flights would be cancelled on Friday affecting 72,000 passengers.
It said it was "deeply regretful" about the negative impact on customers.
The pilot walkout comes after negotiations over pay and work schedules - which began last month - broke down.
Most domestic, European and all long-haul flights have been cancelled. The strike does not affect flights operated by SAS partners, which make up approximately 30% of all departures.
Customers who have a flight booked, are advised to check if their flight is affected by the strike.