Ukraine’s parliament is to decide whether to bring in martial law as anger over the capture of three of its naval vessels by Russia spilled into the streets overnight.
Protesters gathered outside the Russian embassy in the capital Kiev and an embassy car was set on fire.
The incident, in waters off the Crimean Peninsula, marks a major escalation of tension between the two countries.
The UN Security Council will hold an emergency meeting as a result.
Each country blames the other for the incident in which two gunboats and a tug were captured and a number of Ukrainian crew members injured.
There have been growing tensions between the two sides over access to the Black Sea and the Sea of Azov off the Crimean peninsula – annexed by Russia in 2014.
Ukrainian forces have been fighting Russian-backed separatists in the east of the country – but this is the first time the two militaries have come into open conflict in recent years.