Ukraine's former president and current lawmaker Petro Poroshenko has said "today is a tragic day" but that Ukraine will prevail.
He was speaking outside parliament as MPs hold crisis talks in Kyiv.
Poroshenko, who was in office from 2014-19, also compared Russian President Vladimir Putin to a modern-day Hitler.
The situation in Ukraine's parliament is tense, and there are many armed people inside, a Ukrainian journalist in the building tells the BBC.
