Russian spy agencies began preparing for the invasion of Ukraine as far back as June 2021, says a report by the Royal United Services Institute (Rusi).
The Federal Security Service (FSB) has quickly dominated populations in occupied areas of Ukraine, Rusi adds.
The report was compiled using sources including captured documents, it says.
Ukrainian intelligence officials, intercepted communications and on-the-ground research also contributed.
Researchers say the FSB has been able to download government computer hard drives to identify pro-Kyiv individuals that its agents want to arrest and interrogate.
Electronic warfare units have been deployed to cut off occupied parts of Ukraine from the outside world to help them enforce control, Rusi says.
According to the think tank, the head of Russian foreign intelligence, the SVR, told President Vladimir Putin they needed more time to prepare and asked for the invasion to be delayed. His request was denied.