Ukraine's president on Monday imposed martial law after Russia fired on and captured three Ukrainian vessels in the Kerch Strait near Crimea.
The country's parliament approved his request to grant him emergency war time powers in the areas considered most vulnerable to invasion, including the ability to requisition private property and suspend elections for 30 days beginning on Wednesday. He had earlier asked for 60 days across the whole of Ukraine.
In a televised address to the nation, he said the move was necessary to defend Ukraine from a possible Russian ground invasion and promised that a presidential election scheduled for March would go ahead as planned.
The move is controversial - martial law has never been imposed before, even during the height of the war in eastern Ukraine in 2014. Three former presidents of the country issued a statement opposing the move on Monday.
Russia meanwhile defied calls from Western countries to release the Ukrainian vessels and 24 crew members seized in the Black Sea on Sunday.
Two modern gunboats, the Nikol and the Berdyansk, and a tug boat, the Yana Kapu, were rammed, fired on, and ultimately boarded by Russian special forces after they attempted to sail through the Strait of Kerch on Sunday.
The vessels were en route from Odessa to Mariupol, a Ukrainian industrial and military port on the Azov Sea. Russia temporarily blocked the passage under the Kerch Bridge, which links Russia and Crimea, to prevent their passage.
Russia said the Ukrainian vessels had not informed the border control of their plans and that coast guards opened fire after warning to stop "dangerous" maneuvering in closed waters around Crimea.
Russia annexed Crimea from Ukraine in 2014. The FSB, Russia's internal security service, said a criminal case had been opened.
Ukrainian officials have repeatedly denied any wrongdoing by the crew members and described Moscow's actions as military aggression.
Tensions in the Sea of Azov have been mounting since Russia completed the bridge between its mainland and Crimea earlier this year.
Dmitry Peskov, a spokesman for the Kremlin, said the incident that the Ukrainian ships illegally entered Russian waters and called it "a dangerous provocation" organized by Kiev.
Theresa May said in statement that the incident was "further evidence of Russia's destabilising behaviour in the region and its ongoing violation of Ukrainian territorial integrity."
US President Donald Trump said he did not like what was happening between Russia and Ukraine and was working with European leaders on the situation.
Mike Pompeo, US Secretary of State, called Russia's seizure of the Ukrainian vessels "a dangerous escalation and a violation of international law" and called for restraint from both countries.
"The United States condemns this aggressive Russian action. We call on Russia to return to Ukraine its vessels and detained crew members, and to respect Ukraine's sovereignty and territorial integrity," Mr Pompeo said in a statement.
The State Department said Mr Pompeo spoke by phone with Mr Poroshenko and reiterated strong US support for Ukraine's sovereignty and territorial integrity in the face of Russian "aggression".
Angela Merkel, the chancellor of Germany, spoke with Mr Poroshenko by telephone. Her office said the chancellor emphasized the need for de-escalation and dialogue and said she would work to help bring both about.
At an emergency mission of the United Nations Security Council, Nikki Hayley, the US ambassador to the United Nations, called Russia's behaviour "arrogant" and a "violation" of international law, adding: "The United States will continue to stand with the people of Ukraine against Russian aggression."
In Kiev, scores of protesters gathered outside the Russian embassy setting tires on fire and throwing smoke grenades. A crowd of nationalist protesters also gathered outside the Russian consulate in Kharkiv.
The whereabouts of the 24 Ukrainian crewmen was unknown on Monday evening.
The FSB released images of the Ukrainian vessels docked in the Crimean port of Kerch on Monday morning. Sergei Aksyonov, the head of Russian-annexed Crimea, said in a Facebook post that three crew members who had been injured were provided with medical care and their lives were not in danger.