Russian Foreign Minister Sergey Lavrov warned Wednesday that if a third world war were to take place, it would involve nuclear weapons and be destructive, according to Russian state-owned news agency RIA Novosti.

Jailed Russian opposition leader Alexei Navalny is urging people in Russia and around the world to stage daily protests against Russia's invasion of Ukraine.

"We -- Russia -- want to be a nation of peace. Alas, few people would call us that now," Navalny said Wednesday in a series of posts on Twitter via his spokesperson. "But let's at least not become a nation of frightened silent people. Of cowards who pretend not to notice the aggressive war against Ukraine unleashed by our obviously insane czar."

"They say that someone who cannot attend a rally and does not risk being arrested for it cannot call for it. I'm already in prison, so I think I can," he tweeted. "We cannot wait any longer. Wherever you are, in Russia, Belarus or on the other side of the planet, go to the main square of your city every weekday and at 2 pm on weekends and holidays."

"Yes, maybe only a few people will take to the streets on the first day. And in the second -- even less," he added. "But we must, gritting our teeth and overcoming fear, come out and demand an end to the war. Each arrested person must be replaced by two newcomers."

Navalny called on people to not just "be against the war" but to "fight against the war."

"If in order to stop the war we have to fill prisons and paddy wagons with ourselves, we will fill prisons and paddy wagons with ourselves," he tweeted. "Everything has a price, and now, in the spring of 2022, we must pay this price. There's no one to do it for us."

Navalny, the most prominent critic of Russian President Vladimir Putin and the Kremlin, was imprisoned last year when he returned to Russia from Germany after recovering from an attempted assassination with nerve agent poisoning in Siberia. Russia has denied carrying out such an attack.

Ukrainian President Volodymyr Zelenskyy warned Wednesday that the international community "can't stay neutral," as the Russian invasion entered its seventh day.

"Neutral Switzerland has supported EU sanctions against Russian oligarchs, officials, the state, and companies. Once again - neutral Switzerland. So why do other countries wait?" Zelenskyy said in a televised address. "Our anti-war coalition has already been joined by those countries that Moscow was counting on a week ago. This is an extraordinary result. You can't stay neutral right now."

"We are in our homeland and there will be an international tribunal for waging the war against us," he added.

Zelenskyy also praised his fellow Ukrainians for being "united."

"During this time, we have truly become one," he said. "Today you, Ukrainians, are a symbol of invincibility. A symbol that people in any country can become the best people on Earth at any moment."

Russia claimed Wednesday to have captured Ukraine's southern port city of Kherson, the largest Ukrainian city to fall to Russian forces since the start of the invasion.

"Russian forces have taken full control of the Kherson regional center," Russian Ministry of Defense spokesman Igor Konashenkov said at a press briefing Wednesday.

Russia-backed separatist forces in eastern Ukraine have also seized several cities and towns, advancing nearly 100 miles since launching the offensive, according to Konashenkov.

"Ukrainian servicemen will go home as soon as they make a written pledge not to take part in the hostilities," he said.

Meanwhile, Russian forces have "disabled" the instrument room of the the main television tower in Ukraine's capital, Kyiv, "putting an end to information attacks on Russia," according to Konashenkov, who noted that "no damage was done to residential buildings in the course of the strike."

Konashenkov also claimed that Russian aircraft had delivered a "massive strike" on Ukraine's military infrastructure on Wednesday.

"Sixty-seven sites have been hit," he added. "In all, 1,502 elements of Ukrainian military infrastructure have been disabled in the course of the operation. These include 51 command and communications centers belonging to the Ukrainian Armed Forces, 38 S-300, Buk M-1 and Osa air defense missile systems and 51 radar stations."

-ABC News' Anastasia Bagaeva