Hundreds of United Airlines flight attendants on Thursday protested outside several airports against staffing cuts, the first demonstrations by the airline's largest labor group in more than two years.

United is planning to reduce staffing by one flight attendant per flight in its international business cabins and will have meals pre-plated by ground caterers instead of having flight attendants take on that task.

The staffing aboard flights varies depending on the length of the flight and the type of aircraft that is used. They will still be above minimum requirements set by the Federal Aviation Administration, according to the flight attendants' union. But flight attendants argue that fewer crew members on board will put both safety and customer service at risk.

"We need them there," said a 20-year United flight attendant who was picketing outside Newark Liberty International Airport and declined to give her name. Protests were planned outside 16 different United hub airports Thursday. "They're our eyes. It's about safety."

United's CEO Oscar Munoz said the reductions would bring the airline more in line with those of its main rivals, American and Delta.

"Change is difficult," Munoz said on a call with reporters Wednesday to announce new international routes and new flights from its hub in San Francisco. "We'd been staffed higher than all other of our major competitors on those flights."

United's stock price soared to a record this year as a bold bet on aggressive growth, which initially spooked investors, paid off and the airline posted higher-than-expected profits. United's shares are up 29 percent this year, while Delta's are off 4 percent and American's are down more than 36 percent.

"It is not about taking people out of the system or anything, but we're a competitive financial business as well," said Munoz. "Everyone in the United family is contributing, and this is a way that we're going to ask our flight attendants to do that as well."

United plans to hire about 2,100 flight attendants next year, Munoz said. At the end of 2017, United had 22,676 flight attendants, who are part of the Association of Flight Attendants labor union, according to a company filing.

The demonstrations were the first since before United and the flight attendants forged a new five-year labor contract in 2016. Preparation for negotiations starts next April. United merged with Continental Airlines in October 2010, but the two flight attendant groups worked separate flights until October of this year.

The new contract improved relations between flight attendants and the company, but the staffing cuts, particularly for the choice international flights, have emerged as a new point of tension.

One less flight attendant per flight means one less person to respond to an emergency or to de-escalate a conflict between passengers, said Jennifer Ritter, government affairs committee chairwoman at the union's Newark chapter, United's largest flight attendant base.

"Hopefully they'll take it from us that it's not a good idea," she said.

WATCH: What it's like to train to become a commercial pilot — behind the scenes of United Airlines' simulation hub