I’m not Turkish - and my knowledge is spottier than those who really know a lot. But from what I’ve read (and who knows....) his support is defintely not overwhelming, the last election he “won” by 51%, so that means 49% don’t support him. And if Turkish elections are like the ones here, they’re rife with fraud.
Also, the night/day of the coup, coup supporters were told to stay inside by, so the streets were filled with Erdog supporters. Plus, IMHO the whole thing was generated by Erdog to flush out his opponents and terrify everyone, and to enable him to do exactly what he has done. So IMHO the street thugs who butchered and tortured the soldiers who surrendered (apparently under the impression that the whole thing was an exercise) were already johnny on the spot to do their butchery.
Coup supporters among the population were inside their houses. And good that they were, or they would have received the same treatment.
I don’t know the specifics, but there were a LOT of people on the streets opposing the coup - and Istanbul was supposed to be the base of the coup’s support.