Russian parliament on Tuesday adopted an act prohibiting members of “extremist” organisations, a move allies of Kremlin critic Alexei Navalny say aimed at stopping the members of “extremist” organizations from serving as legislators during parliamentary elections in September.

The bill approved in first reading by a lower house of parliament takes the view that the anti-corruption foundation of Navalny and regional campaign groups are being banned as extremists.

The draft law proposes to prevent any member or group leader declared extremist from taking up the seats in the lower house of the State Duma during periods of between one and five years.

If passed, the bill would likely derail campaigns for the parliament announced by activists such as Lyubol Sobol, who is currently under house arrest for taking part in a protest in support of Navalny.

Sobol described the bill on Twitter as an “unconstitutional” attempt to bar her from running in the election this September.

Before becoming law, the bill needs to be approved in two more readings in the lower house, before being backed by the upper house and signed by President Vladimir Putin.

Vasily Piskarev, a lawmaker from the ruling United Russia party who co-authored the bill, said it was necessary to bar people from “extremist” organizations from holding seats in parliament because they could use their position “for propaganda and the justification of their ideas, as well as for the recruitment of new followers.”

If Russia formally declares Navalny’s Anti-Corruption Foundation and regional groups extremist, authorities will gain the formal power to jail activists and freeze their bank accounts. The next court hearing in the case is set for June 9.

The case has already prompted Navalny’s allies to disband the regional groups.

Navalny’s allies say the extremism allegations are an attempt to blunt their political opposition to United Russia at the parliamentary election.