The fire started at around 8 p.m. at 77 Lafayette Avenue and quickly spread to a third alarm. Mutual aid was called in from several surrounding towns.
"I look outside and see ashes forming on my window," said Michael Fortuna, a witness.
Residents say the fire started in apartment 4B and then spread from there.
It didn't take long for the building to become almost completely engulfed in flames.
The Passaic Fire Chief Patrick Trentacost says that it appears the fire was electrical.
As portions of the roof began to collapse, a crowd gathered across the street, watching firefighters aim their hoses on the rising smoke and flames.
A woman who lived there says she moved in just two weeks ago.
She watched helplessly as her belongings went up in flames.
"Everything is gone, got to be gone, the whole building was on fire," she said.
Two firefighters are being treated for minor burns.
Officials say up to 22 people have been displaced and the apartment building is a total loss.
Displaced residents are at the senior center at City Hall and the Red Cross is on the way to help them.