Greenpeace T-shirts have been banned from Parliament after the organization staged an illegal demonstration on the Hill this week.

The ban is due to "recent events," a security guard told a reporter from The Canadian Press who entered the Parliament buildings as a visitor Thursday wearing a Greenpeace T-shirt.

The reporter put on the shirt to confirm reports that security was searching visitors for Greenpeace logos.

The T-shirt was brought to the attention of nearby officers who told the reporter she could not enter the buildings. They relented after she agreed to turn the T-shirt inside-out.

A House of Commons security guard then issued a stern warning: "We won't tolerate anything. We will charge you."

About 20 Greenpeace activists were arrested Monday after scaling the roof of the Parliament buildings and unfurling a sign attacking Prime Minister Stephen Harper and Liberal leader Michael Ignatieff for their environmental positions.

The protest embarrassed RCMP and other security officials on Parliament Hill.

Heather Bradley, a spokesperson for the Speaker's office, said it is routine for guests to be obliged to remove attire that may "disrupt" proceedings.

"When someone is invited in as a member of the public in either the chamber or a committee, they're invited in as an observer, that's it," she said. "They're not a participant, they're an observer."

Bradley offered the example of several visitors who were recently asked to remove anti-abortion pins before entering the public gallery.

New Democrat MP Libby Davies said she understands Monday's protest may have heightened tensions, but Parliament Hill is a place that should defend freedom of expression.

"The fact is, wearing a T-shirt that says Greenpeace on it does not mean that you're going to do something out of line," she said.

"I think they need to have a measured and reasonable response."