Two children are among at least six people who have been detained or questioned in Russia over online comments viewed as "justifying terrorism" since last week's deadly attack on a concert hall, according to local authorities and Russian media.
Security services have been on high alert since the March 22 gun rampage at the Crocus City Hall near Moscow that killed at least 143 people. The Federal Security Service (FSB) has urged people to tell the authorities immediately if they hear of any calls to carry out attacks in Russia.
Both President Vladimir Putin and the FSB chief have said they believe Ukraine was involved in the attack, for which the Islamic State militant group has claimed responsibility. Ukraine has denied any involvement.