Marseille prosecutor Brice Robin, citing information from the "black box" voice recorder, said the co-pilot was alone in the cockpit.
He intentionally started a descent while the pilot was locked out.
Mr Robin said there was "absolute silence in the cockpit" as the pilot fought to re-enter it.
He said air traffic controllers made repeated attempts to contact the aircraft, but to no avail. Passengers could be heard screaming just before the crash, he added.
Details are emerging of the German co-pilot's past - although his apparent motives for causing the crash remain a mystery.
"We hear the pilot ask the co-pilot to take control of the plane and we hear at the same time the sound of a seat moving backwards and the sound of a door closing," Mr Robin told reporters.
He said the pilot, named in the German media as Patrick S, had probably gone to the toilet.
"At that moment, the co-pilot is controlling the plane by himself. While he is alone, the co-pilot presses the buttons of the flight monitoring system to put into action the descent of the aeroplane.
"He operated this button for a reason we don't know yet, but it appears that the reason was to destroy this plane."
Mr Lubitz was alive until the final impact, the prosecutor said. Mr Robin said "the most plausible interpretation" was that the co-pilot had deliberately barred the pilot from re-entering the cockpit.
He added that the co-pilot was "not known by us" to have any links to extremism or terrorism.
Passengers were not aware of the impending crash "until the very last moment" when screams could be heard, Mr Robin said, adding that they died instantly.
After Thursday's revelations, several airlines have pledged to change their rules to ensure at least two crew members are present in the cockpit at all times.
Meanwhile, relatives and friends of the victims have been travelling to the Alpine region where the plane came down, near the town of Seyne-les-Alpes.
The French authorities have provided them with a viewing tent overlooking the crash site, in a remote mountain ravine, the Associated Press news agency says.
The disclosure of the likely cause of the crash has provoked anger.
"One person can't have the right to end the lives of hundreds of people and families," Esteban Rodriguez, a Spanish factory worker who lost two friends aboard the aircraft, told the Associated Press news agency.
The principal of a German high school that lost 16 students and two teachers in the crash said the latest news was "much, much worse than we had thought".
Residents of Alpine villages near the scene of the crash have also expressed shock.
"For the pilot it's suicide, perhaps, but it's an attack on the other people. Yes, an attack," Charles Bosshardt, a mountain risk adviser, said.
The second "black box" - that records flight data - has still not been found.
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