Political activists gathered in downtown Moscow on the Russia Day public holiday on Tuesday, braving rain showers for an anti-Kremlin rally as its organizers were called in for questioning during the event, according to RIA Novosti.

The March of Millions, which was approved by the authorities for up to 50,000 participants, will see the protesters march from the capital`s Pushkin Square to Prospekt Akademika Sakharova, where opposition figures are due to address the crowd.

The attendance was far below that figure at the march’s start, according to a RIA Novosti correspondent on the scene. The red flags of the Communists predominated, with plenty of nationalists also in attendance. No official turnout estimates were available as of 1 p.m.

"While we are united, we can never be defeated," chanted a crowd composed of hardline Communists, liberal democrats and nationalists, summing up neatly for one brief moment the disparate nature of the anti-Putin movement.

"They will not scare us!" chanted protesters at the first demostration since a dramatic increase in fines for protest-related offences, introduced by a bill signed into law by President Vladimir Putin last Friday.

The rally is the first of its kind since a May 6 event on Bolotnaya Square, which ended with protesters clashing with police and some 650 arrests, according to rights activists. Police put the figure of arrests at around 400.

Thirteen people were later held on charges of starting the riots, and the homes of opposition leaders, including Alexei Navalny, Sergei Udaltsov, Ilya Yashin and Ksenia Sobchak, were searched on Monday in connection with the case.

The protest leaders were also summoned for questioning at 11 a.m. on June 12, an hour before the start of the rally.

Navalny, Yashin and Sobchak obeyed the summons, telling reporters they hoped to make it in time for the round of speeches, set to start at 3 p.m.

Udaltsov opted to go to the rally instead, his lawyers formally requesting the investigators to reschedule the meeting. He led the crowd in chanting “Revolution!” at the start of the march.

Investigators insisted the searches were legal, but opposition leaders said the move was an attempt at political pressure ahead of the rally.

Tensions have mounted in the country after the clashes in May, with police cracking down several times on camps that protesters tried to pitch in the capital’s center.

A bill has been fast-tracked through the federal legislature recently, tightening rules for staging public events and hiking fines for violations of public order at rallies from 1,000 rubles ($30) to 300,000 rubles ($9,000).