The strike is causing inconvenience to the passengers
India's second-largest private airline Jet Airways has cancelled nearly 120 flights, following a protest by hundreds of pilots who reported sick.
At least 35 flights from the financial capital Mumbai and 18 flights from Delhi have been cancelled.
Hundreds of passengers have been stranded at various airports, leading to angry confrontations.
The pilots are protesting against the dismissal of two senior pilots last month by the airline.
The pilots' union, the National Aviator's Guild, has been demanding the reinstatement of the pilots.
"Jet Airways has been in a conciliation process with the pilots. The Regional Labour Commissioner had categorically stated that any strike by pilots during this (period) would be deemed an illegal act," news agency Reuters quoted from a statement issued by the airline.
"Regrettably, a section of the pilots has resorted to a simulated strike by reporting sick," it said.
The strike would "damage the airline's operations and inconvenience the travelling public", the statement added.
A spokesman for the pilots' union said the protest would continue until their demand was met.
"All we want is that they give us in writing that the two sacked pilots will be reinstated. Management also has some responsibility in meeting us half-way," Reuters quoted spokesman Girish Kaushik as saying.
The strike has led to the cancellation of flights from airports across India.
The airline cancelled at least six flights from the southern city of Madras and four from Calcutta in the east.
The airline has been mired in controversies in recent months. Last year, it announced plans to lay off 1,900 staff to cut costs.
But after days of protests by employees, the government persuaded the airline to take them back.
India's once-booming aviation sector has been hard hit by soaring costs, mainly due to global fuel-price rises.