Moscow is bracing for more mass demonstrations this weekend, as tensions between city authorities and opposition activists increase over protests demanding free elections. After last weekend’s demonstration, which drew some 10,000 protesters and saw nearly 1,400 arrested, city authorities and opposition leaders are showing no signs of compromise. Officials in the capital have opened a criminal investigation into the unsanctioned rally and detained protest leaders, who have called for another demonstration on Saturday. Moscow's mayor, Sergei Sobyanin, has made his loyalty to the Kremlin clear. In an interview with the local TV Centre television channel on Tuesday, he described Saturday’s gathering as “mass disorder well planned in advance.” He said some protesters had attacked law enforcement officers and “simply forced the police to use force.” Mr Sobyanin then issued what might have been intended as a warning to those planning to protest this weekend: "Anarchy, disorder and lawlessness make real problems worse and end in tragedy. Order will be maintained." Local parliament elections in September are at the centre of the standoff between authorities and the opposition. Elections officials have barred some 57 opposition or independent candidates from running, claiming they failed to collect enough genuine signatures needed to appear on the ballot. Ahead of planned protests on Saturday, most of the prominent protest leaders have been detained. Vladimir Milov, a former deputy energy minister and aid to opposition leader and anti-corruption activist Alexei Navalny has been sentenced to 30 days imprisonment violating protest laws. Mr Navalny himself is serving the same sentence for calling for last weekend’s demonstration. Konstantin Yankauskas, another candidate, is serving seven days for his role organising last week’s rally. That has placed the spotlight on the only barred candidate still at large, Lyubov Sobol. In an interview with the <em>Moscow Times </em>website this week, Ms Sobol doubled down, saying now is the time for "radical measures." The aide to Mr Navalny is the most high-profile of the barred candidates. She has launched a hunger strike in protest against the decision to bar her from running, and was recently filmed being carried out of the elections office on the sofa where she tried to stage a sit-in. “We will come out every Saturday in the centre of Moscow until they put us on the ballot,” Ms Sobol said. Moscow elections are typically an unremarkable local affair, but a growing chorus of international voices have criticised the crackdown on protesters. On Wednesday, the United Nations joined the EU, the US and rights groups by voicing concerns of “excessive” police force. UN human rights spokesman Rupert Colville on Tuesday cautioned that the “use of force by the police should always be proportionate to the threat, if there is one.” The hospitalisation of Mr Navalny with an unexplained illness over the weekend has heightened the drama. On Monday, his lawyer said the rashes and skin lesions he developed in police detention were the result of “poisoning.” The Kremlin faces a dilemma in how to respond to the prostest, said Tatiana Stayanova, a political scientist writing for the Carnegie Centre. As long as the administration fails to come up with solutions to the protests, she says, security forces will be forced to act. “The risk is that opposition activists will not be the only ones persecuted; those in power deemed soft on the ‘internal enemy’ will be too,” she said. Writing in the independent <em>Vedomosti</em> business daily this week, columnist Pavel Aptekar warned of the risk of security services being given free rein to end the growing protest movement. “Batons and violent arrests are capable of quickly radicalising the mood of Muscovites and other cities,” he wrote, warning more police violence could, “trigger something unpredictable.”