On Sunday 27 February at 7:15 AM GMT, the BBC reported that: (source)
Ukraine asks foreign volunteers to help
Kyiv says it's establishing an "international" legion for foreigners wishing to volunteer in support of Ukraine. "This will be the key evidence of your support for our country," remarked President Volodymyr Zelensky in a statement issued just now about Ukraine's new "foreign legion".
The use of the word "legion" suggest that these volunteers would become combatants. Would it be legal for European citizens to join such a Ukrainian foreign legion, and take up arms in the Russia/Ukraine conflict? Would they have to wait until their own government formally gives permission to its citizens to join the Ukrainian military (like the Irish/British situation in WWII)? Would they risk being prosecuted when returning home, if they joined now without awaiting such a decision? And what would their status be if they were captured by Russian forces?
In a later article, the BBC quotes the Ukrainian president making the claim that it would be legal for foreign nationals to volunteer:
A more detailed post says foreign citizens are legally allowed to join Ukraine's defences, and that a separate division of fighters is being formed called "the International Legion for the Territorial Defence of Ukraine".