Thousands of employees of India's state-run domestic airline have begun an indefinite strike, disrupting air travel across the country.
The merger is the latest shake-up in the Indian aviation sector
Indian Airlines says some flights have been cancelled and most are delayed.
Baggage handlers, check-in staff and other employees of Indian Airlines want better wages and career prospects.
The strike action came after the government approved a plan to merge the struggling airline with the country's flagship carrier, Air India.
The two have faced increased competition from new private airlines in recent years and Indian Airlines, also called Indian, has seen its market share drop to 20%.
The BBC's Sanjoy Majumder in Delhi says staff fear that the merger with Air India may eventually lead to job cuts or hurt their future prospects.
The strike began on Tuesday night after a breakdown in talks with management.
There have been chaotic scenes in some of India's major airports, including the capital, Delhi, the financial capital, Mumbai (Bombay), and the IT hub of Bangalore.
Although flights operated by private airlines were mostly on schedule, long queues of prospective travellers have been reported.
Passengers say there has been little information on when services will return to normal, or what to do until they do.
Neelesh Verkhedkar, whose wife was booked to travel on an Indian Airlines flight out of Mumbai, told the BBC: "We saw in the morning news that the flights have been delayed or cancelled and since she has to be in Nagpur tonight, we came to the airport to make alternate arrangements."
He said they managed to get her a ticket on a private airline but the lack of information from Indian Airlines was "frustrating".
"They haven't even put up a list of which flights have been cancelled. Everything is very uncertain and we couldn't take any chances."
Another passenger at the airport said he would "never fly Indian Airlines again".
"These private airlines offer much better service," businessman Anand Kumar Kanodia told the BBC.
Indian Airlines operates more than 300 flights a day, carrying some 30,000 passengers.
By about midday (0630 GMT), just 40 flights had departed while 12 had been cancelled, Reuters news agency reported.
The airline's website contains no special advice for passengers seeking to travel. But a spokesman said: "We are deploying staff from other places to keep operations running."
The government has suspended 23 strikers and is threatening action against those who do not return to work.
It has been a frustrating time for passengers across the country
Civil Aviation Minister Praful Patel called the strike illegal.
He said was not in the interests of staff or the airline and employees should understand that in times of competition, passengers have other options.
The merger of Indian Airlines with state-owned international carrier Air India is aimed at making them able to compete more effectively with private carriers.
Set up in 1932, Air India is the country's flagship international airline, serving more than 40 destinations worldwide.
Indian Airlines is focused on the domestic market, but recently began some flights to destinations in Asia and the Middle East.
In the last few years, privately owned rivals such as Jet Airways and Kingfisher Airlines have taken market share from the duo.