In a formal and dignified manner. (Oxford Dictionaries)
In a serious manner. (Oxford Dictionaries)
With deep sincerity. (Oxford Dictionaries)
Grave, sober, or mirthless, as a person, the face, speech, tone, or mood: (Dictionary.com)
Serious or earnest: (Dictionary.com)
Seriously and without any humour. (Cambridge Dictionary)
Marked by the invocation of a religious sanction. (Merriam – Webster)
marked by the observance of established form or ceremony. (Merriam – Webster)
In solemn Paris ceremony, Macron leads global WW1 Armistice commemorations.
Solemn ceremony of the start of production.