Relatives of people killed in the Germanwings plane crash have gathered in Cologne for a memorial service.

Hundreds of dignitaries, including German chancellor Angela Merkel, also attended the service.

The steps to the altar of the cathedral were covered with 150 lighted candles, one for each person who died - including co-pilot Andreas Lubitz, who investigators believe deliberately crashed the plane.

"It's not for us to judge," Cardinal Rainer Maria Woelki, the archbishop of Cologne, told Bild newspaper ahead of the service about the decision to include a candle for Lubitz.

Most victims of the March 24 crash in France were German or Spanish. Flight 9525 was en route from Barcelona to Duesseldorf.

French and Spanish ministers were among some 1,400 people at the memorial service, which was carried live on German television.

Lufthansa, Germanwings' parent airline, took out full-page advertisements in many of the country's leading newspapers expressing sympathy and carried a live-stream of the service on its website.

Flags were ordered flown at half-mast around the country as part of the tribute.

Cardinal Woelki told the relatives of the victims that words alone were too weak to give them any solace, but that they should take comfort in the numbers of people with them at the memorial service, and those following it online or on television around the country.

"You are not alone in these hours of loneliness," he said.

More than 80 per cent of the debris from the plane crash in the Alps has now been recovered and removed.

Though prosecutors believe Lubitz intentionally crashed the plane, they are still trying to determine why.