Madonna fans have been warned against "a threat of physical violence" by the US Embassy in Moscow as the Russian leg of the singer's MDNA tour gets under way.
The embassy said in a statement: "The US Consulate General in St Petersburg has received information regarding a threat of physical violence against spectators and performers at the St Petersburg concert on August 9.
"Russian authorities have indicated to the Embassy that they are taking appropriate measures in light of this information.
"While we expect that enhanced security measures will be put in place at both concerts, US citizens are reminded to remain vigilant regarding their personal security, and to be aware of their surroundings at all times, especially in crowded areas."
It comes after Madonna weighed into the row over imprisoned Russian punk band Pussy Riot.
During a show in the Russian capital on Tuesday, the Like A Virgin singer donned a trademark Pussy Riot balaclava and told the crowd she hopes the three band members will be released soon.
The crowd at Olimpiyskiy stadium cheered their support for Madonna when she said: "I know there are many sides to every story, and I mean no disrespect to the church or the government, but I think that these three girls - Masha, Katya, Nadya - I think that they have done something courageous. I pray for their freedom.
"I know that everyone in this auditorium, if you are here as my fan, feels they have the right to be free."
The 53-year-old performer also stripped to her bra to reveal the band's name written on her back.
Maria Alyokhina, 24, Nadezhda Tolokonnikova, 22, and Yekaterina Samutsevich, 29, all face up to three years in jail for storming the altar of Christ the Saviour Cathedral on February 21 and singing a "punk prayer" calling on the Virgin Mary to rid Russia of President Vladimir Putin.
The women have already spent five months in custody on charges of hooliganism.
Lead singer Tolokonnikova has compared their trial to Russia's Stalinist era, calling it a "political order for repression (that meets) the standards of Stalinist troikas". She also predicts "the collapse of this political system".
Other international performers such as Sting and the Red Hot Chili Peppers have echoed Madonna's support for the band.
The case is seen by the opposition as part of a wider crackdown on dissent by Mr Putin following the biggest protests since he rose to power in 2000. It has seen a toughening of the rules that govern internet access and increasing fines for protestors.
Pussy Riot's stunt was aimed at both Mr Putin and the head of the Russian Orthodox Church, Patriarch Kirill, who backed him during his recent presidential campaign.
Many Russians have been angered by the reaction of the state and church, calling it disproportionate.
The three women may be sentenced this week.
