More than 1,000 British Airways pilots have marched on BA's Heathrow headquarters over plans to use non-BA pilots in a new subsidiary.
British Airways plans to fly to the US direct from other European cities
BA's "OpenSkies" service is due to fly from non-UK European capitals to the USA, following an EU-US agreement.
Pilots had voted for industrial action, but BA applied for a legal injunction to prevent it going ahead.
British Airways says the new airline would not be viable if all its pilots were employed under existing terms.
Referring to the members who turned out for the protest, Captain Dave Smith of the British Airline Pilots Association (Balpa) said: "This shows that we are determined that our jobs and our security and our family lives will be defended."
A spokesman for the association said given that roughly 2,000 of BA's 3,000 pilots are usually either on duty or resting before or after flights, the turnout was very high.
"Every pilot who could be there was there, which was quite remarkable," the spokesman said.
Balpa General Secretary Jim McAuslan said: "This march and rally comes a week after a BA threat to serve an injunction on our association. Our message is clear - we will not be bullied, we will not be brushed off, we will persevere."
Protesters wore caps and T-shirts bearing the slogan 'BA planes - BA pilots' and a small plane carrying a banner flew over London as part of the demonstration.
When Balpa balloted its members for strike action earlier this year, 86% voted in favour.
BA has said it will use Article 43 of the Treaty of Rome to defend its OpenSkies plan. The article allows companies in the EU freedom to establish business anywhere.
"Pilot unions and other unions around the world realise that if BA was to be successful in the courts that would have massive ramifications for every union," Mr McAuslan said.
"We have asked the High Court to pronounce on BA's attempt to use this law in an industrial dispute," Mr McAuslan said.
British Airways said it does not want conflict with its pilots, and believes the opportunity remains to reach a peaceful resolution and lift the threat of strikes.
The airline said in a statement it was disappointed that Balpa has "chosen to walk away" from talks at a conciliation service.
"We have offered binding assurances that OpenSkies will pose no threat to the terms, conditions and job security of British Airways pilots," the company said, adding that it has offered BA pilots 50% of flight crew vacancies for the six planes planned for the new airline.
"It remains our view that the issues between us can be resolved without the need for a strike, which would be a wholly disproportionate response to our creation of a European business that will have no adverse impact on BA pilots."