Detained Zimbabwe protest pastor Evan Mawarire says he's being well treated in the maximum security prison in Harare where he's being held - but state media called him a "charlatan" who was past "sell-by date".

His #ThisFlag protest movement said in a press release on Sunday: "He is so grateful for all the prayers, support and encouragement and has asked that we reassure you that he is at peace and has been treated well during his detention."

Mawarire, 39, was arrested on Wednesday as he landed in Zimbabwe after six months in exile, much of it spent in the US. He's been charged with insulting the national flag and trying to overthrow President Robert Mugabe's government. His lawyers say he did not try to stage a coup and only called for non-violent means of protest.

In an opinion piece for the state-controlled Sunday Mail, pro-Mugabe writer Lovemore Mataire said Mawarire was just a "commonplace" guy and a "charlatan" who managed to gather just a "handful" of sympathisers before leaving Zimbabwe in July last year.

Though it appears many Zimbabweans are still weighing up whether to support Mawarire in 2017, his popularity in 2016 is less debatable. The success of a nationwide strike in early July led to his arrest - and even Mugabe criticised him.

Said #ThisFlag: "He is well and in good spirits."