Lil Wayne -- Party Evacuated After Pepper Spray Attack

Lil Wayne was attacked with pepper spray last night in Canada -- after some jerk snuck into his afterparty and put the excruciating substance in the air ducts.

It all went down last night at a club in Montreal called La Mouche -- where Weezy was hosting the afterparty for his show.

We're told some jackass snuck into the back room and unleashed the chemical into the ventilation system -- and everyone had to be evacuated, including Weezy.