Relations between Kiev and Moscow have soured since 2014 when Crimea rejoined Russian Federation following a referendum, but Ukraine said the peninsula was illegally annexed by Russian Federation and remains a Ukrainian territory.
"The president's website has sustained an organised attack".
"This is yet another example of the ease with which President Poroshenko unjustifiably tries to control public discourse in Ukraine", said Tanya Cooper, Ukraine researcher at Human Rights Watch.
The decision was described in a decreeposted on the presidential website as part of economic sanctions against Russian Federation, which annexed Crimea from Ukraine in 2014 and has sent weapons, equipment, and troops to support a fuel the separatist side in a war in eastern Ukraine.
The decree imposes a ban on access to popular Russian social media networks, such as VK (formerly VKontakte) and Odnoklassniki, both owned by the Mail.Ru Group.
The attackers were using resources of Yandex and Vkontakte to create "artificial traffic in order to overload and bring down the servers", deputy head of the presidential administration, Dmytro Shymkiv, said in a statement posted on Facebook.
"It is an absolutely historic day for Ukraine, for my 45 million nation, and I am absolutely confident that this is a historic day for the European Union", Poroshenko said after the signing.
As the share of Ukrainian revenue is immaterial, the company doesn't plan to review its revenue guidance. Yandex said that it does not see the sanctions as having a material negative impact on its consolidated results.
In Russia, the Kremlin spokesman Dmitry Peskov called Poroshenko's decision "another manifestation of unfriendly, short-sighted policy towards Russia".
Ukraine has imposed new sanctions on Russian social media and web services, affecting millions of its citizens and leaving many users anxious about their data. In the list, except for Vkontakte, Classmates got, Mail.ru, Yandex and others.
Moscow has repeatedly denied accusations from Kiev that it has been waging a "cyber war" against Ukraine.
Conflict broke out in eastern Ukraine as pro-Russian separatists launched an insurgency against the government in Kiev.
Other Russian companies banned in Ukraine include airline Aeroflot, Russian Helicopters, and state arms exporter Rosoboronexport.