One of Canada's largest public sector unions on Tuesday alleged political interference after police ordered a small plane towing a banner reading "Stephen Harper hates us" to land.

The Royal Canadian Mounted Police told public broadcaster CBC the weekend flight over Ottawa strayed into restricted airspace, so the pilot was ordered to land for questioning.

"The RCMP undertook questioning of the pilot in order to determine if there was a threat to Parliament Hill," Corporal Lucy Shore said to the Canadian Broadcasting Corporation.

But the Public Service Alliance of Canada said it believed the RCMP's actions were a politically motivated attempt to silence critics of Prime Minister Stephen Harper's government.

The union has balked at massive public sector layoffs, and paid for the plane pulling the banner, according to the pilot.

"Had the RCMP been concerned about the plane's potential route, it could have easily communicated with the pilot via radio and sought clarification rather than ordering it back down to the airport," the union said in a statement.

Pilot Gian Piero Ciambella has denied breaching restricted airspace around Canada's parliament and the prime minister's official residence.

He told CBC: "They were worried. They weren't sure who I was, what it was all about."

He said officers warned him the anti-Harper message might constitute hate speech banned under Canadian law.

Neither the union nor the RCMP were immediately available for comment.

Harper spokesman Carl Vallee denied the prime minister's office was involved in any way. "It was an operational decision made by the RCMP," he told AFP in an email.

[Image via Agence France-Presse]