The Ukrainian president, Petro Poroshenko, has banned the country’s two most popular social networks, its most popular email service and one of its most widely used search engines as part of sanctions against Russian companies.

A decree by Poroshenko posted late on Monday expanded sanctions adopted over Russia’s annexation of Crimea and backing of separatists in eastern Ukraine to include 468 companies and 1,228 people. Among them were the Russian social networks VK and Odnoklassniki, the email service Mail.ru and the search engine company Yandex, all four of which are in the top 10 most popular sites in Ukraine, according to the web traffic data company Alexa. The decree requires internet providers to block access to the sites for three years.

Poroshenko’s decree also blocked the site of the Russian cybersecurity giant Kaspersky Labs and will ban several major Russian television channels and banks, as well as the popular business software developer 1C.

In a post on his official page on VK, Poroshenko said he had tried to use Russian social networks to fight Russia’s “hybrid war” and propaganda. But Russian cyber-attacks, including its alleged interference in the French election this month, “show the time has come to act differently and more decisively”, he said, declaring he would shut down his pages on the networks.

Asked about the possibility of counter-sanctions, Vladimir Putin’s spokesman told journalists that he wasn’t prepared to say but that Russia had not “forgotten about the principle of reciprocity”.

The sites were still working on Tuesday, but the national security and defence council said it had ordered the cabinet of ministers, security service and national bank to develop a mechanism to stop access to them. The major internet provider Ukrtelecom told the news site Novoye Vremya that it had begun the process of blocking the sites, which could take several days.

Mykhailo Chaplyga, the representative of the Ukraine parliament’s commissioner for human rights, told the news agency UNIAN that “blocking access to sites without a court decision is not allowed” under Ukrainian law. Web industry representatives said it would take time and investment to put the ban into place, and users would be able to get around it easily. Already, VPN clients are popular in Russia and Ukraine as a means of access to sites blacklisted by the authorities.

VK, also known as VKontakte, told the Russian site TJournal that the “internet by its nature doesn’t have borders” and promised to defend the interests of its users. Odnoklassniki promised it would find a way for Ukrainians to keep using the network.

Several Ukrainian politicians spoke in favour of the ban amid the simmering conflict with Russian-backed separatists, which has killed at least 10,000 people since 2014. One MP, Volodymyr Ariev, argued that the social media networks were a security risk since Russian intelligence has access to their data and could gather information about Ukrainian users, including state employees and soldiers.

But many internet users began ridiculing the ban almost immediately. Of more than 11,000 respondents to an online poll on the UNIAN site on Tuesday afternoon, 66% said they were “categorically against” the ban of VK, Yandex and other Russian sites. Another 11% said it would be easier to “ban the whole internet, like in North Korea”.

Some laughed that Poroshenko was withdrawing Ukraine’s “armchair warriors” from foreign social media, while others joked that Kiev had decided to ignore its transport problems by blocking Yandex and its traffic jam monitoring. “By banning VK and Odnoklassniki, we’re basically admitting our inability to counter Russian propaganda in cyberspace,” one blogger wrote in a post on the InformResist site. Many users in Russia and Belarus began listing how much they would charge to post photos to Ukrainians’ social media accounts.

Ukraine has previously blacklisted Russian cultural figures and films and declared the French actor Gérard Depardieu persona non grata. In recent months, it has banned the American actor Steven Seagal for being a threat to national security and forbade Russia’s Eurovision entrant from taking part in last weekend’s song contest.