Thousands joined last Friday's climate rally, which was also attended by Swedish activist Greta Thunberg.
After the main rally, some activists, reportedly from Austria and Germany, blocked one of Vienna's main roads, leading to 92 arrests.
Videos that went viral after the demonstration show a police offer hitting a protester who had been pinned to the ground, and a police van almost running over the head of another man while he was being restrained by officers.
The Vienna prosecutors' office said it was investigating four police officers for a range of alleged offences including “grievous bodily harm” and promised a “quick and objective probe into the different accusations”.
Five witnesses had been questioned as part of the inquiry, it added in a statement.
The Vienna police department said on Tuesday it was probing the allegations – with one officer concerned having been removed from street patrols.
However, it described some of the allegations levelled at police as “absurd” and insisted that “officers had to use force to make the arrests”.
Controversy over police violence is rare in Austria.
A demonstration against police brutality has been called for Thursday.