The Council of Europe's human rights commissioner has called on the Russian government to abide by international treaties on the right to free assembly.
A letter from commissioner Thomas Hammarberg, posted on the Council of Europe's website on Friday, says the right to gather peacefully is "clearly stated" in several international and regional treaties ratified by Russia, including the European Convention on Human Rights (ECHR) and the Universal Declaration of Human Rights.
It is also enshrined in Article 31 of the Russian constitution, the letter points out.
Opposition activists have gathered in a central Moscow square on the 31st day of the month that has one to highlight this, but the rallies are frequently dispersed by police.
Hammarberg also regretted that there were no provisions in Russian law to allow for "spontaneous assemblies."
"The ability to hold such events is important because a delay would weaken the message to be expressed," the letter says.
He also expressed concern that police often exceed their authority when dealing with protest rallies.
"Force has often been used - at times excessively - and participants in assemblies have been apprehended and brutally treated by the police, even during peaceful events," the letter said.
"Authorities should ensure that law-enforcement officials at all levels respect and protect the right to freedom of assembly and act lawfully... Appropriate training in the policing of public assemblies incorporating human rights principles should be provided on a regular basis to such officials."
Last year, a journalist had his arm broken by riot police during a rally in Moscow calling for Prime Minister Vladimir Putin to stand down.
Hammarberg also said police should be held accountable for any "illegal act" they commit during protests.
The commissioner also expressed concern at arrests made after protests, referring specifically to the 15-day detentions of opposition leaders Boris Nemtsov and Eduard Limonov following a rally on New Year's Eve last year.
He said any person charged over a protest should have a "fair trial."
Russia's permanent representative to the Council of Europe, Igor Kapyrin, said the Russian authorities acted within the law in dealing with rallies and protests.
The letter comes just three months before Russia holds a crucial parliamentary election, followed by a presidential vote in March next year.