British Airways, the only airline using Terminal 5, cancelled 34 flights due to what it called "teething problems." There were also long baggage delays, with some passengers reporting waits of up to four hours. Computers at the terminal were reportedly unable to cope with the amount of bags at the airport.
The airline said in a statement on its website that, "It is extremely disappointing that this should happen on the first day of our operations from Terminal 5."
Both domestic and international flights were hit, with services to Munich, Frankfurt, Paris and Brussels as well as Glasgow and Aberdeen grounded.
Some 200 demonstrators wearing 'Stop Airport Expansion' T-shirts had protested the opening of the new terminal, as well as plans for a sixth terminal and a third runway at the world's busiest airport.
There have been a number of security breaches by protestors at the site of the new terminal recently. One of the most high-profile of these occurred when a man sprinted onto the runaway as Queen Elizabeth II opened the terminal earlier this month.
