Grieving relatives joined political and religious leaders Friday at a sombre German memorial service for the victims of last month's Germanwings crash in the French Alps, blamed on a depressed co-pilot.

Flags flew at half-mast nationwide for the 150 dead during the ecumenical service at Cologne's historic cathedral attended by German Chancellor Angela Merkel and President Joachim Gauck along with ministers from France and Spain.

Co-pilot Andreas Lubitz, who had been diagnosed as suicidal in the past, is believed to have deliberately flown the plane into the mountainside after locking the pilot out of the cockpit.

He 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.

Gauck said the tragedy had left the country in a state of "enormous shock" and that Lubitz's alleged role only compounded the suffering of the victims' families.

"Train drivers, ship captains and pilots must all be people we trust -- they bear the responsibility for the lives of many people," he said.

"If this trust in such a sensitive area is abused, it strikes us to the bone. As we heard the terrible news, we also sensed it could have been any of us."


'150 victims'

A white flag emblazoned with a black cross hung outside the cathedral, while in front of the altar 150 candles were lit, one for each of those killed.

The service at northern Europe's largest Gothic church was broadcast live on screens outside the cathedral to an assembled crowd and to viewers nationwide.

The archbishop of Cologne, Cardinal Rainer Maria Woelki, and the head of the Protestant Church of Westphalia, Annette Kurschus, led the service.

Ahead of the ceremony, Woelki urged compassion for all the dead, including Lubitz.

"There are 150 victims," he insisted.

Media reports said that Lubitz's parents had been invited to the service but declined to attend.

Woelki said the national ceremony was intended to assure the bereaved that the country shared in their pain.


'Absolute security' impossible

Aviation industry doctors have demanded in the wake of the crash that German pilots undergo more extensive medical checks, while several airlines worldwide have changed rules to require two crew in cockpits at all times.

Gauck stressed that "absolute security" was impossible but noted that the disaster had prompted a national debate "about mental illness and its consequences" and "the responsibility that those affected bear".

Lufthansa chief Carsten Spohr, a former pilot, attended the ceremony with three executives of its low-cost subsidiary Germanwings.

Spohr, who is grappling with a heavy blow to the airline's image, asked pilots in attendance not to wear their uniforms to the ceremony.

Relatives accepted small wooden angels representing the victims and wept as opera singer Luiza Fatyol performed "Pie Jesu", a frequent part of requiem masses.

Mourners left flowers and lit candles on the stairways leading to the cathedral, and outside the city's main railway station nearby.

The Germanwings Airbus 320 was en 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 devastated by the loss of 16 students and two teachers from a high school in the small town of Haltern as they returned from a class trip to Spain.

A youth ensemble from the school played the mournful theme to the 1993 film "Schindler's List" during the service.

Business executive Peter Eiglmeier said he had driven to Cologne from the northern city of Hamburg to take part in the public show of sympathy.

"I lost two children myself a few years ago. My thoughts go out to the parents of those kids on the plane," the 57-year-old told AFP, fighting back tears.

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, where the passengers embarked.