And more of these...
Back in the day many classically trained opera singers did do also these popular songs, sometimes under a pseudonym. Georg Malmsten of course effectively switched completely to popular music, while e.g. Ture Ara and Sulo Sala did both. Sulo Sala in general seems to be mostly forgotten nowadays (apparently there were no major hits for him), a shame since what I have heard of him I have liked.
And then again there were singers with no classical training, like A. Aimo.
Here Ture Ara performs a traditional Karelian folk song in early 30s, with an interesting mixture of bombast and melancholy:
Ture Ara: Reppurin laulu / Vallinkorvan laulu
And a split single from mid-40s,
A. Aimo / Sulo Sala: Ruusuja hopeamaljassa / Suo mulle hymysi Maria