TALLINN, ESTONIA -- Alexei Navalny鈥檚 team is used to working independently. The most potent foe of Russian President Vladimir Putin was frequently absent for long stretches after being arrested, assaulted, poisoned, or imprisoned.

But when Navalny died suddenly in February at age 47 in a remote Arctic prison, his team was left with a monumental challenge: sustaining an opposition movement against Putin 鈥 who is all but assured to be reelected 鈥 without the living example of their defiant and charismatic leader.

After the initial shock wore off, Navalny's closest allies returned to the work that cost his freedom and his life: undermining Putin鈥檚 iron-fisted grip on power.

A significant test will come Sunday, the last of three days that voters can go to the polls in an election that is widely viewed as more of a formality than an exercise in democracy.

That's when Navalny鈥檚 team 鈥 with the endorsement of his widow, Yulia Navalnaya 鈥 is calling for a protest dubbed 鈥淣oon Against Putin.鈥 They are asking Russians to flock to polling stations Sunday at noon local time across the country's 11 time zones to demonstrate their discontent with Putin鈥檚 rule and his war against Ukraine.

鈥淚t is a very simple and safe action, it can鈥檛 be banned,鈥 Navalnaya said in a video address. 鈥淚t will help millions of people to see their like-minded allies and to realize that we are not alone, we鈥檙e surrounded by people who are also against the war, against corruption and against lawlessness.鈥

Navalny鈥檚 followers have expressed a wide mix of emotions in the weeks since his death, from renewed inspiration to a sense of defeat.

Maria Obukhova of Moscow, who paid tribute to Navalny on Wednesday at the Borisovskoye Cemetery, said the crowds she saw at his funeral 鈥 which numbered in the thousands 鈥 were motivational.

鈥淚t was a huge surprise for me, because it seemed before like everything had died here, that Russia is no longer, that it had died,鈥 said Obukhova, who placed white daisies at Navalny's gravesite.

Valery, another Muscovite at the cemetery, who withheld his last name for security reasons, said he had little hope for the future and that after Navalny's death, that 鈥渟omething has really broken鈥 inside of him.

Just several days after her husband's death, Navalnaya expressed determination to keep his mission alive.

In the past month, she has addressed the European Parliament, met with United States President Joe Biden, and urged Western countries not to recognize the results of Russia鈥檚 election. She has also called on the West to impose more sanctions on those close to Putin.

Leading up to the election, Navalny鈥檚 team urged supporters to cast their ballots for any candidate other than Putin, or to invalidate them by choosing two or more candidates. They also had dozens of volunteers call ordinary Russians to ask them about their grievances and try to turn them against Putin.

The phone campaign was announced by Navalny over the summer, and since then 鈥渢ens of thousands鈥 of calls were made, Leonid Volkov, Navalny鈥檚 top strategist, said a video. 鈥淲e will not stop doing that,鈥 he vowed.

Volkov also gave a video address shortly after Navalny鈥檚 death in an effort to rally supporters, and perhaps tap into his longtime ally鈥檚 spirit of persistence. 鈥淚t will be a monument to Alexei鈥檚 cause if you and I live to see how this regime disintegrates before Putin鈥檚 eyes,鈥 he said.

Still, the Putin opposition's uphill battle has only gotten steeper with its leaders in exile.

鈥(Putin鈥檚) regime pushes people out of the country because it understands very well that the possibilities of influencing political processes in Russia from abroad are minimal,鈥 said Nikolay Petrov, a visiting researcher at the German Institute for International and Security Affairs.

Sunday's 鈥淣oon Against Putin鈥 protest will be a test of how much Navalny's team can do in Russia from abroad, said Sam Greene, a director at the Center for European Policy Analysis in Washington.

鈥淥ne part of what they want to do is to send a message to those who remain in Russia that you鈥檙e not alone, that the opposition in exile has their back to a certain extent and will support them," said Greene. "But then the question is, how do they support them?鈥

Efforts are underway to disrupt the protest. Navalny's team said fake emails have been sent around purporting to be from them telling Putin opponents to show up at the polls at 5 p.m. instead of noon.

Russia鈥檚 independent election watchdog, Golos, reported that officials in at least one region are being instructed to report large gatherings near polling stations to the police.

On Thursday night. the Prosecutor's Office in Moscow warned that unauthorized rallies near polling stations 鈥渕ay prevent citizens from freely exercising their voting rights and the work of election commissions,鈥 a criminal offense punishable by up to five years in prison.

The personal risks for Putin's opponents remain high.

On Tuesday, Volkov was attacked near his home in Lithuania. Assailants smashed a window of his car, sprayed tear gas into his face and beat him with a hammer, according to Navalny鈥檚 team.

Volkov was taken to a hospital, and upon release said his arm was broken and his leg was injured so much it was painful to walk.

He accused 鈥淧utin鈥檚 henchmen鈥 of the attack and said it was an attempt to intimidate the team ahead of the 鈥淣oon Against Putin鈥 protest.

With Navalny gone, some of his supporters are recalibrating their expectations.

Valery, one of many people who visited Navalny's grave in southeastern Moscow in recent weeks, said he is less optimistic about the opposition's prospects going forward.

鈥淓ven though Yulia, his wife 鈥 his widow 鈥 has picked up the baton, I鈥檓 not sure that it is going to be the same as it was when Alexei was alive," he said.