TORONTO - Canadian security agents and police have been harassing and intimidating grass-roots activists in an effort to stifle legitimate opposition to this month's G20 summit in downtown Toronto, community groups said Tuesday.

The groups said they had tracked 28 incidents over the past three months in which security officials or police had contacted organizers at home or work, showed up at their meetings, or followed them around their neighbourhoods.

Some agents accused organizers of being "un-Canadian" or of supporting violence, the activists said, adding they would not be silenced by the tactics.

"It's a very fine line to be drawn between people who are planning violent activities and people who are planning non-violent expressions of their political views," said Kevin Tilley, of the Summit Legal Project.

"If the police contact is expanding beyond simply interfering with people who are planning violence and including people who are planning non-violent political expression, then there's a very big problem in terms of interfering with people's democratic rights to organize."

As dozens of police officers looked on, the groups made their complaints at a news conference outside the Metro Convention Centre, where the summit will be held June 26-27.

In response, the Canadian Security Intelligence Service said agents are simply carrying out their mandate to assess potential threats.

"CSIS is continuously assessing the potential for violence resulting from the activities of certain groups and individuals leading up to, during and after the summit," said spokeswoman Isabelle Scott.

She noted no one is compelled to talk to a CSIS agent, and people should not read anything into a particular request for an interview.

To ensure the security of world leaders and prevent violent protests, authorities are mounting an unprecedented show of force costing hundreds of millions of dollars, and turning an area around the convention centre into a fenced-off no-go zone.

Activists said they would not be deterred from using the G20 summit to express opinions on, or raise questions about, a range of issues — among them poverty, the environment, and aboriginal and gender rights.

"Our questions are going to be met with $1 billion worth of wire fencing, concrete, surveillance, armed men and women, snipers, weapons of war, police, soldiers, spies, riot vans, batons, tear gas, hand guns, rubber bullets (and) a propaganda machine that labels every Torontonian as criminal, suspect, menace, vandal or terrorist," said community worker Greg Thomas.

"This is Toronto, not Tiananmen Square," he said in reference to the lethal crushing of protesters in Beijing in 1989.

The Integrated Security Unit, which is the lead agency handling summit security, did not immediately return calls seeking comment.

The organizers also accused the media of portraying the situation as "protesters versus police" instead of as "the people" confronting their political leaders.

While an area has been set aside at the Ontario legislature for demonstrations, police have said they will not attempt to confine protesters to the that area.

Members of the Toronto Community Mobilization Network said they were planning a series of events and demonstrations under the title of "themed days of resistance" and "days of action."

"We have to build a movement of resistance to the G20," said Liisa Schofield of the Ontario Coalition Against Poverty.

"We will be bringing the demands of our communities and the anger at the conditions that people are forced to live in every single day."

Each planned event is accompanied with an organizer's assessment — low, medium or high — of perceived level of police interference.