Such actions of Moscow fundamentally contradict universally recognized provisions and principles of global law and grossly violate the United Nations Convention on the Law of the Sea of 1982 and its bilateral treaty obligations to Ukraine, the Foreign Ministry said.
It is noted Putin first talked with the builders of the bridge.
The Verkhovna Rada of Ukraine officially declared February 20, 2014, the date of the beginning of the temporary occupation of Crimea by Russian Federation.
"At last, thanks to your talent, this project, this miracle, has happened", Putin told the workers. This was written by the President of Ukraine Petro Poroshenko on his page in Facebook on Tuesday, may 15.
In Kiev, Ukrainian President Petro Poroshenko said: "The illegal construction of the Kerch bridge is the latest evidence of the Kremlin's disregard for worldwide law".More news: Anushka Compliments Deepika For Her Cannes Look
"It is particularly cynical that its opening is happening on the eve of the latest anniversary of the deportation of the Crimean-Tatar people by the Stalin regime".
The Crimean Bridge, at 19 kilometres (nearly 12 miles) will be Europe's longest, overtaking Lisbon's Vasco da Gama Bridge.
The EU has imposed a range of sanctions over Russia's illegal annexation of Crimea in 2014, carried out in retaliation for Ukraine ousting its pro-Russian president in what Russia has condemned as a Western-backed coup.
The bridge's opening ceremony drew further criticism.
"The illegal construction of the Kerch Strait Bridge is another indication of the neglect of worldwide law by the Kremlin".More news: North Korea details plans to dismantle nuclear bomb test site
Meanwhile, Ukraine's security service raided the offices of Russian state media in Kiev and detained one journalist, moves the chief editor of Russia's RT television suggested were linked to the bridge opening.
"The European Union continues to condemn the illegal annexation of Crimea and Sevastopol by Russian Federation and will not recognize this violation of global law", the spokesperson said.
The Russian leader, who was re-elected for a fourth Kremlin term in March extending his long rule, then pledged to build more of "such projects" across Russia.
The Kremlin said the bridge would be opened to cars on Wednesday.
Ukraine, which along with most of the global community has not recognised Russia's annexation of Crimea, condemned the project, which was personally championed by Putin. "Many didn't believe these plans were possible", Kremlin spokesperson Dmitry Peskov told reporters on Tuesday ahead of the ceremony.More news: Tesla Driver in Utah Crash Says Autopilot Was Engaged
"Today, both in a practical sense and from a symbolic point of view, is an extremely important day", he added.