I am not aware of any permits for anti-war/anti-military-intervention demonstrations to be held legally in Russia. I have been monitoring Russian and Ukrainian social media since December 2021, when the preparations for the invasion started escalating rapidly. As you mentioned, illegal actions have been taking place, often limited to single people. In most cases, the participants have been prosecuted by the government.
Protests against the Russian invasion of Ukraine started February 24, 2022. They take place as demonstrations, one-person pickets and other social actions in Russia as well as in other countries. In Russia, anti-war protests are banned and are suppressed by the police; many of the participants are prosecuted for criminal and civil crimes. According to the human rights organization OVD-Info (ОВД-Инфо), altogether from February 24 until May 9 in russia 15441 people have been detained for participation in anti-war protests. - Protests against the Russian invasion of Ukraine - Wikipedia (in Russian)
Below are a few recent tweets from from the Twitter account of the human rights organization OVD-Info (ОВД-Инфо):
[Municipal representative] Elena Kotyonochkina during the session of the council of representatives called Russia "a fascist state" and, following the reports filed by informants, left the country. Elena is a colleague of Ilya Yashin and Alexey Gorinov. A criminal proceeding about "fake [information dissemination]" was filed against all three of them." - OVD-Info, July 22, 2022
In Moscow police detained Anna Mikhaylova, who participated a one-person picket with the poster "No war", as well as Vladislav Gur'yanov, who made photos of the woman.
OVD-Info, July 22, 2022
An activist from Ivanovo was detained for a single person picket - he held the poster with "*** *****" written on it. The detained man was charged with a new law about "discrediting Russian military".
(Note that the words on the poster could be interpreted as "net vojne" ("no war" in Russian))
Maxim Drukovsky. TJournal, March 12, 2022