Slabs of concrete and metal crashed to the floor of departure lounge 2E early on Sunday trapping people who were waiting for flights and spreading the rubble over 30m of the lounge. It is unclear why the roof collapsed.
France Info radio reported six people had been killed, saying interior ministry officials had given them the figure. Firefighters at the airport said four were killed.
A Reuters photographer reported ambulances ferrying people to hospital while fire engines rescue workers searched for survivors feared trapped under the rubble.
"Several tonnes of concrete collapsed in a waiting area in a departure lounge," said firefighter Captain Laurent Vibert.
He said a red alert had been declared and a large number of emergency vehicles attended.
Police kept reporters out of the terminal while emergency services worked. A nearby terminal, 2F, was still in operation, he said.
Terminal 2E was inaugurated last year. It is used by national carrier Air France and several other international airlines.