Germany will hold a memorial service Friday to mourn the victims of the Germanwings crash, blamed on a depressed co-pilot believed to have deliberately steered the plane into a mountain.

Flags will fly at half-mast nationwide for the 150 victims as political and religious leaders join hundreds of bereaved relatives at a noon (1000 GMT) service at the imposing Cologne Cathedral.

German Chancellor Angela Merkel and President Joachim Gauck will attend the ecumenical service along with Spain's Interior Minister Jorge Fernandez Diaz and French Transport Minister Alain Vidalies.

Lufthansa chief Carsten Spohr will also attend, as will three executives of its low-cost subsidiary Germanwings, Thomas Winkelmann, Axel Schmidt and Oliver Wagner, the company said.

About 1,500 guests are expected for the service, among them 500 relatives of the victims, in northern Europe's largest Gothic church, which will also be broadcast live on screens outside the cathedral and to viewers nationwide.

Mourners will be able to leave flowers and light candles on the stairways leading to the cathedral, and outside the city's main railway station nearby.

A white flag bearing a black cross hangs outside the cathedral, while in front of the altar 150 candles have been set up, one for each of those killed.

The Germanwings Airbus 320 was on route from Barcelona to Duesseldorf when it crashed in the French Alps on March 24, killing everyone aboard, including 72 Germans and 50 Spaniards.

Germany was especially moved by the loss of 16 students and two teachers from a high school in the small town of Haltern as they returned from a trip to Spain.

Co-pilot Andreas Lubitz, who had a history of depression, is believed to have deliberately flown the plane into the mountainside while the pilot was locked out of the cockpit.

Lubitz was receiving treatment from neurologists and psychiatrists who had signed him off sick from work a number of times, including the day of the crash.

Aviation industry doctors have since demanded that German pilots undergo more extensive medical checks, while a string of airlines have changed rules to require two crew in cockpits at all times.

The memorial service will be led by the Archbishop of Cologne, Cardinal Rainer Maria Woelki, and the president of the Protestant Church of Westphalia, Annette Kurschus.

Loved ones of victims previously attended a memorial event near the disaster site, at the village of Le Vernet in the French Alps.

On April 13, the foreign ministers of Germany, Spain and France also paid tribute to the victims at Barcelona airport.

Cologne Cathedral was constructed over more than six centuries between 1248 and 1880. Despite several hits, it survived Allied bombing that levelled much of Cologne in World War II. It was listed as a UNESCO World Heritage site in 1996.