Police staffing in several Russian cities is so stretched as officers are deployed to bolster security at World Cup venues that a union leader has warned criminals could benefit.
Several police officers in cities across Russia said their staff were working long hours, patrols had been reduced and response times to incidents had slowed.
"The situation is very dangerous .... This could lead to grave consequences," Vladimir Vorontsov, who represents Russia's Inter-regional Police Trade Union, said.
"You could get to a situation where there are simply not enough police to do the work, and the only people who benefit from that are the criminals."
Russia has deployed thousands of police to the 11 cities hosting matches to deal with potential trouble and other security threats such as the risk of terrorism when the tournament starts on Thursday.
Alexei Lavrishchev, the head of the Main Operations Control Centre for Security at the World Cup, said on Wednesday that police were not overstretched and sufficient numbers were available to secure the tournament and cities not hosting the World Cup.
And world football's governing body, FIFA, said it had confidence in the Russian authorities.
"FIFA has complete trust in the security arrangements and comprehensive security concept developed for the 2018 FIFA World Cup by the Russian authorities and the LOC [local organising committee]," a FIFA spokesperson said.
"As demonstrated during the FIFA Confederations Cup last year, the high security standards already in place in Russia have been adapted to meet the specific needs of such major sporting events."
A senior regional police officer told Reuters that officers were on call 24 hours a day and, even when not working, needed permission to leave their accommodation. He said the working day typically lasted around 14 hours.
"The food is dreadful, the conditions are spartan," the officer said.
Vorontsov said rank-and-file officers were "completely wrung out, there's a big staff shortage, no one wants to join, the treatment of subordinates is inhuman."
But two other police officers said detectives and special units were working effectively.