Canada's largest public sector union is accusing the RCMP of trampling on free speech in grounding a plane carrying an anti-Harper banner over Ottawa, and warned the plane and the banner will fly again.

The Public Service Alliance of Canada had contracted a pilot to fly a 2,000-square-foot banner written in French, which reads "Stephen Harper nous déteste.ca" (translated as "Stephen Harper hates us.ca" in English).

The RCMP said officers requested the plane land on Saturday because it was in restricted airspace, but PSAC regional vice-president Larry Rousseau said he believes the plane was grounded because of the message.

"There was a message in the sky and the message was brought down. And that constitutes an attack on freedom of speech. That's what we're calling it," said Rousseau.

The grounding of the plane carrying the banner over the weekend "was an operational decision by the RCMP, not a political one," wrote Carl Vallée, a press secretary for Prime Minister Stephen Harper, in an email.

Pilot says RCMP were worried

Pilot Gian Piero Ciambella said he first started flying the banner in Montreal in mid-August during the city's gay pride celebrations and garnered a lot of interest. He has since flown it elsewhere in Quebec over a two-week period.

Ciambella took the banner into Ottawa-Gatineau skies on Saturday.

"I was supposed to cover Parliament Hill and Ottawa-Gatineau area, and after almost two hours of flight I was intercepted, where I was asked to come on down for a security check from RCMP officers," he told CBC News by phone Tuesday morning.

"They intercepted me because they had thought that I was flying inside the no-fly zone, which is .35 nautical miles from the Peace Tower. The radar … in the Ottawa control zone, my communications with them made sure that I was not in that particular restricted area. I never really penetrated that."

Ciambella said he told the officers that he was simply doing his job advertising a website for a legal Canadian organization.

"They were worried. They weren't sure who I was, what it was all about," he said. "They considered that as Stephen Harper hates us, our prime minister hates us. They're in charge of Parliament Hill security and the security of the prime minister, so they reacted based on that. They didn't see the .ca I guess, basically, which explains what it was all about. They maybe jumped the gun too early."

An email statement sent Monday from RCMP spokeswoman Cpl. Lucy Shorey said the plane "appeared to be within restricted airspace" when the officers spotted it.

"As such, the RCMP requested the plane to land," Shorey wrote. "The RCMP undertook questioning of the pilot in order to determine if there was a threat to Parliament Hill. It was deemed that no threat was being posed. The matter is now concluded."

Freedom of speech issue, says union

Rousseau told Ottawa Morning host Robyn Bresnahan Tuesday that the RCMP officers who questioned Ciambella said the sign might constitute "hate speech."

"In this case, if you are critical of the prime minister or of the government or of the government's policies, you should have the complete right to be able to say that," Rousseau said Tuesday.

"What we do outside of the workplace is freedom of speech. This is purely a freedom of speech issue and that's why this union is doing what unions do so well," Rousseau said. "We're raising a little hell about this because we should have complete freedom under the Charter to criticize the prime minister, and that's what we're doing when we say Stephen Harper hates us."

Rousseau said over the next couple of weeks — weather permitting — the plane and its anti-government banner will be making a return visit to the skies over the capital.