Flags were flying at half-mast nationwide for the 150 dead as political and religious leaders were to join hundreds of bereaved relatives at a midday (1000 GMT) service at Cologne's historic cathedral.
German Chancellor Angela Merkel and President Joachim Gauck will attend the ecumenical service in the western city 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.
Ursula Mund, 53, said she would be among those watching on the large square in the city centre.
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"Of course this is a special occasion. We have all talked a lot about what happened and are still baffled by it," she said.
"We are still saddened and I feel very moved today."
Michael Senker, 62, said he would follow the ceremony on television.
"It's important to me to watch because all of Germany has been particularly affected by this tragedy," he said.
Mourners began leaving flowers and lit candles on the stairways leading to the cathedral, and outside the city's main railway station nearby.
A bouquet of a dozen white tulips left at the foot of the towering cathedral had a card bearing the message, "Depression is incalculable," referring to co-pilot Andreas Lubitz who medical records show had a history of the illness.
A white flag bearing a black cross hung outside the cathedral, while in front of the altar 150 candles were set up, one for each of those killed.
Cologne Cardinal Rainer Woelki insisted on compassion for all the dead, including Lubitz.
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.