Two of the people are Vladivostok judges and one is an official of the Russian Federal Security Service (FSB).

"We are talking about renewing the so-called Magnitsky list, which enables to enforce personal visa sanctions against people who have helped to violate human rights with grave consequences," Jevgeni Krištafovitš, one of the authors of the public letter and head of the NGO Center for European Initiatives, said in a press release. "In this case, the grave consequence is taking a person's freedom for fighting for his political beliefs. We turned to Sven Mikser because the Republic of Estonia is the first member state of the European Union that adopted the Magnitsky law in 2016," he added.

Tretyakov, 28, was arrested at the end of March on suspicions of inciting extremism online after he published the writing of the exiled journalist Arkady Babchenko on the popular Russian messaging application Telegram about an opposition protest event. A court in Vladivostok took Tretyakov into custody and at the beginning of April the Primorsky Regional Court dismissed an appeal against his arrest.

"We, the signatories of the address, are convinced that the real reason behind the persecution of Mister Tretyakov is his public activity, including as a lawyer of the Primorsky Krai base of politician Alexei Navalny. Therefore we are applying for visa sanctions to be enforced against FSB official Denis Makarenko as well as district judge Aleksandr Rybakov and krai judge Elena Danilochkina," Krištafovitš said.

"With that we want to show all the people around the world who commit human rights violations that the will not escape responsibility only because they are operating in a different part of the world, far from large centers. Every politically motivated decision or order which results in grave consequences for dissidents will not go unnoticed and without international sanctions. That's why the Magnitsky list was created," he added.

The address has been signed by 71 civil society activists connected to Russia, most of whom have had to leave Russia due to political persecution and are currently living in other European countries.

The list with the names of the 49 people on whom an entry ban was imposed by the Estonian government on March 29 on the basis of the International Sanctions Act, also known as the Magnitsky list, took effect in Estonia on April 3, although the Estonian parliament already in December 2016 adopted a law amendment which would enable to prohibit high-ranking Russian officials who are on the so-called Magnitsky list from entering Estonia.

This legal basis for the application of an entry ban has been dubbed Magnitsky provision after the case of Russian lawyer Sergei Magnitsky, who exposed large scale tax fraud committed by Russian interior ministry officials and died in unclear circumstances in the Butyrka pretrial prison in Moscow in 2009 at age 37.