Addressing officials and legal scholars at the Kremlin, Medvedev suggested the proposed amendment would prepare the country to confront 21st-century challenges.
"The Constitution is a fundamental document," Medvedev said. "But that does not mean that we can't look at the Constitution through the eyes of modern people, people who live in the 21st century."
Critics deride the amendment, quickly approved by the State Duma last month, as a ploy to further strengthen the Kremlin's grip over the country and pave the way for Prime Minister Vladimir Putin to return to the Kremlin for a 12-year rule.
The amendment must now be passed by at least two-thirds of the country's regional parliaments -- a process likely to be a mere formality.
During Medvedev's speech, youth opposition activist Roman Dobrokhotov began heckling the president, calling the amendment "shameful."
When Medvedev began talking about human rights, Dobrokhotov, leader of the youth group We, shouted that human rights are not respected in Russia and that the country has no civil society, Gazeta.ru reported.
A brief scuffle ensued between Dobrokhotov and security guards, who tried to usher the activist from the hall. Medvedev then asked them to leave Dobrokhotov alone.
"Really, you don't have to take him away, let him stay and listen," Medvedev said, Interfax reported. "To be honest, the purpose of the Constitution is to allow everyone to voice his opinion."
The audience applauded Medvedev's comments, but the security guards removed Dobrokhotov from the auditorium anyway, news agencies reported.
It was unclear how the activist managed to get into the event, given that security at such speeches is typically very strict. Dobrokhotov's cell phone was switched off all day both Friday and Sunday.
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