Activists targeted the public utility headquarters because of public hearings being held on the future of Gentilly-2, Quebec's sole nuclear facility.
Protesters want the facility to be closed.
Two other activists were also detained after climbing onto a small roof over Hydro-Quebec's doorway.
All six arrested in Montreal face trespassing charges.
There were two more arrests at public hearings held by the Canadian Nuclear Safety Commission in Becancour.
One man was arrested after he shoved a pie in the face of a hydro official attending the meeting.
A second man was taken into custody after yelling that the hearings were illegitimate.
Activists suit up in biohazard gear for protest.
Shortly after 6:30 a.m., activists dressed in white Hazmat suits took over Hydro-Quebec's entrance on Rene-Levesque Boulevard.
They placed about 20 bright yellow barrels with radioactive warning symbols around the entrance, and four people chained themselves to the front doors.
Two others climbed onto the overhang above the front steps to hang banners calling for the closure of Gentilly-2.
Around 8 a.m., Montreal police used bolt cutters to break the chains and arrest the activists. Firefighters used a ladder truck to remove the other protesters from their perch above the entrance.
Greenpeace Quebec argued the facility is too costly and too dangerous to stay open, pointing to the situation at Japan's troubled Fukushima Daiichi plant to illustrate the dangers of nuclear energy.
Gentilly-2 currently delivers about three per cent of Quebec's overall power, said Greenpeace nuclear analyst Shawn-Patrick Stensil.
"We already have a surplus of electricity. We don't need it. We are exporting electricity. Why live with the risks of a Fukushima-type accident when we don't even need the electricity?"
In Becancour, the CNSC is reviewing Hydro-Quebec's request for a five-year license renewal and massive investment to refurbish Gentilly-2.
Quebec's power utility is willing to spend $2-billion to ensure the facility will remain viable for another 30 years.