A vote that could allow Russia's Vladimir Putin to serve two more terms as president began on Thursday (June 25).
If passed, the constitution would be changed to let him run for two more back-to-back six-year terms after the current one ends in 2024.
Opposition groups have called the ballot a 'constitutional coup'.
Alexei Navalny is a leading Putin opponent.
"What I want to say: do whatever you want, the most important - do not recognize (the vote). You want to vote 'no', to go and vote? For me, such a strategy seems to be a bit exotically naive, even funny. But if your heart is set on it - go and vote 'no'. Just do not recognize, because this is a hoax."
A number of changes to the Russian constitution, which was first written in 1993, are proposed.
Former Russian presidents would be automatically granted immunity from criminal prosecution when they leave power.
Russia would be able to ignore international court rulings seen to contravene the text of its new constitution.
It would enshrine a reference in it to "belief in God" and one reform would define the institution of marriage as the union of a man and a woman.
While critics call the vote a 'sham', the Kremlin argues it is needed to strengthen the role of parliament, improve social policy, and lock in stability.
The ballot is due to last seven days.