The U.S. Embassy cautioned U.S. citizens to exercise increased vigilance ahead of a large opposition rally scheduled for Bolotnaya Ploshchad on Monday evening.
The warning came as Moscow authorities warned that the rally might be canceled over the death of a volunteer on the square but then said it would be allowed to proceed.
City Hall has given the green light for up to 30,000 people to rally from 7 p.m. to 9:30 p.m. against President Vladimir Putin and commemorate an anti-Putin rally held on the same square a year ago. The May 6, 2012, rally, held on the eve of Putin's inauguration to a third term as president, ended in violent clashes that the protesters and the police have blamed on each other.
"The potential for confrontation cannot be ruled out. A larger-than-usual police presence will likely be in and around downtown Moscow to monitor the events," the U.S. Embassy said in a statement.
"U.S. citizens should also be aware of the possibility for counter-demonstrations or other spontaneous protest activity throughout the Moscow region and other urban areas of Russia," it said.
The embassy also warned of possibly significant traffic and mass-transit disruptions on Monday in Moscow and noted that the U.S. State Department already advises U.S. citizens to avoid all public demonstrations, large crowds and public gatherings that "lack enhanced security measures" in Russia.
A volunteer died on the square Monday when sound equipment that he was installing for use at the rally tumbled down on him, Interfax reported. The volunteer was identified as 26-year-old Maxim Melkov by news portal Life News. Interfax put his age at 25.
City Hall initially said the rally might need to be canceled to allow investigators to conduct an inquiry into the death. But later it said the rally could go ahead as planned.
Police said anti-terrorism measures would be in affect, and people would be forbidden from entering the square with sharp items, large bags and plastic or glass bottles containing liquids. Even water bottles will be confiscated, they said.
"Any attempt to engage in an illegal activity will be prevented in strict accordance with the applicable law and the perpetrators will be held accountable," police said in a statement carried by RIA-Novosti.
Police advised participants to arrive early to avoid long lines at the metal detectors.
Opposition organizers had also wanted to stage a march, but City Hall rejected the request, citing the fact that Monday was a working day and a march might interfere with traffic.