Ukraine's first president, Leonid Kravchuk, has slammed Russia's invasion of Ukraine and said he would take up arms to defend his homeland if Moscow does not retreat.
In an op-ed written for Russian website snob.ru, Kravchuk said that Putin had gone too far.
"They have gone beyond their base and occupied Crimea – it is conquered territory," he said.
"Russia violated the Budapest Memorandum. Ukraine has every reason to go to international arbitration."
As the Soviet Union neared its end, Kravchuk signed an agreement that faciliated the birth of the Ukrainian nation and became its first independent president. He held office from 1991-94.
He warned that Russia's actions could provoke a catastrophic conflict in Europe.
"I call on the Russian authorities to stop. Between our peoples should not be war. Does Russia not understand that this is the beginning of World War III?" he warned.
He vowed to defend the land of his forefathers despite his age.
"My great-grandfather and grandfather fought in World War I, along with Russia. I am 80 years old, but I'll take a gun and I will defend their land. Every citizen will defend their territory as their home.
"We knelt and Russia sat us down. Czechoslovakia, Poland, Afghanistan, Hungary. Now Ukraine is on the line?" he continued.
In a sideswipe at Putin, he suggested that the Russia had acted at the "behest of some schizophrenic". He said Russia's elite was governing for itself and not the Russian people.
"People in power are governed by their own ambitions and not the interests of the people," he continued.
"Let Russia think about the relationship of our peoples, rather than protecting power. Presidents come and go, and people are living and will live," he concluded.
China has supported Russia's position in the crisis unfolding in Ukraine after talks between Russian foreign minister Sergei Lavrov and his Chinese counterpart, Wang Yi.
Ukraine's new government, which took over when Viktor Yanukovich was topped from power by mass demonstrations in Kiev and western Ukraine, has accused Russia of declaring war with the invasion.
Foreign secretary William Hague confirmed that Russia had operational control of the Crimea region and called the invasion the "biggest crisis of the 21st century".