Police had earlier believed there were as many as four gunmen, as shots reportedly continued to be heard. Officers continue to secure the area and look for students.
Officials said up to eight of the victims are in critical condition.
CTV's Jed Kahane said police received a call about the shooting just before 1 p.m. ET. Witnesses heard shots at about 12:45 ET that lasted for 30 minutes.
A police officer outside the college saw the gunman enter the building, and police quickly arrived at the scene. The suspect was shot dead as he tried to leave the school.
Panicked students described a scene of chaos and violence, as people fled or hid themselves from the shooter.
"He shot right at us. And when he shot at us we jumped and ran the other way," said student Ali Hussein. He added that one bullet struck a wall close to where he was standing.
Dawson College is located at the corner of Atwater and Sherbrooke in the heart of downtown Montreal.
Students told Kahane they saw someone roaming the halls with a gun, and heard at least 20 shots fired.
One student told Montreal radio station 940 News she was on the phone at the college's front entrance when she heard five gunshots and a window break. She walked into the hallway and was inches from the gunman.
"All of a sudden I turned around and saw a man dressed in black with a huge assault rifle," she said.
"People didn't know what was going on ... they thought it was a joke."
The man ran into the corner of the cafeteria to hide from police, she said.
A number of officers surrounded the school with guns drawn, while others helped to evacuate students from inside the English-language CEGEP school which has about 10,000 students.
Gary Clemence, a psychology teacher, said the college is "usually a really quiet, peaceful place. No problems, no knives, anything."
Interim Liberal Leader Bill Graham said he was shocked and saddened by the shooting, and told Canadians to reach out to the victims.
"We must unite as a country to show our compassion for those whose lives have been dramatically altered by this inexplicable event," he said in a statement.
Montreal Mayor Gerald Tremblay echoed Graham's thoughts.
"This is so tragic. How do we talk to the parents who are going through this? All I can say is that I feel for them, and I care for them," he told Newsnet.
Today's incident is horrifyingly reminiscent of another school shooting in Montreal. On Dec. 6, 1989, Marc Lepine shot 27 female engineering students at the Ecole Polytechnique, killing 14, before fatally shooting himself.
The mass murder prompted tighter gun laws, which included the creation of the controversial national firearms registry. It also prompted Parliament to create the National Day of Remembrance and Action on Violence against Women in 1991, to coincide with the anniversary of the tragedy.
During Wednesday's shooting, some students and teachers barricaded themselves in classrooms, waiting for police to rescue them.
As many as 40 students and staff were hiding on the seventh floor, including eyewitness Adam Perez, who spoke to CTV Newsnet.
"No one came to really warn us," said Perez. "Our first warning of the incident (came from) phone calls and text messages."
One student told 940 News she saw two people who had been shot, including one who had been hit in the neck.
The student said a friend told her four people had been shot.
Michel Boyer, a student at the college, told CTV Newsnet he saw the gunman in a hallway leading to the cafeteria.
"I saw the gunman who was dressed in black and at that time he was shooting at people. It was probably one of the most frightening moments of my life," Boyer said.
Images captured from a helicopter hovering over the scene showed hordes of students running frantically from the building.
"Many people are crying as they are coming out, worried about who might still be inside the building and shocked by what happened," said CTV's Genevieve Beauchemin, reporting from the scene.
A number of police vehicles surrounded what appeared to be a bloodsoaked sidewalk outside the school, and several yellow ambulance vehicles were seen speeding from the scene after victims were carried from the building on stretchers and loaded in the vehicles.