''We only need a small spark for the situation to develop in a way that will be completely out of control for the authorities.''
It's make-or-break time, because on Wednesday a raft of new laws came into effect that make almost everything the protesters have been doing illegal.
The laws, which were rushed through the Ukrainian Parliament last week on a show of hands, ban helmets, hard hats and masks at rallies, and impose fines and prison sentences for setting up unauthorised tents, stages or sound systems in public places.
They prescribe jail terms for anybody blockading public buildings, and make it a crime to ''slander'' public officials (whatever that means). You can also go to jail for handing out pamphlets, and you can get 15 years for being part of a ''mass riot'' (however the Government chooses to define that).
If President Viktor Yanukovych's Government tries to enforce these laws on the tent city of protesters that has filled the ''Maidan'' (Independence Square) since late November, there will be something like civil war in the heart of the Ukrainian capital.
He hasn't done so yet, but cellphone users near the violent clashes early Tuesday morning got text messages saying: ''Dear subscriber, you are registered as a participant in a mass riot.''
Mr Yanukovych is getting desperate, because the protests are no longer just against his abrupt decision not to sign a treaty creating closer trade and political ties between Ukraine and the European Union, and to turn to Russia instead for loans totalling $US15 billion ($NZ18 billion) and discounted gas.
The protests have expanded to take in the dire state of the economy, Mr Yanukovych's ruthless political tactics and the sudden wealth of the ''Family'' of officials and businessmen who support him.
So long as the conflict was about the EU-or-Russia issue, Mr Yanukovych could count on the backing of the Russian-speaking half of the Ukrainian population, in the south and the heavily industrialised east of the country: many people there fear for their jobs if the Ukrainian economy integrates with the EU.
But the poverty and the corruption hurt everybody, whether they speak Ukrainian or Russian. Everybody can get together and protest about that.
Another worry for Mr Yanukovych is the attitude of the oligarchs, the billionaire businessmen such as Rinat Ahmetov, Viktor Pinchuk and Igor Kolomoisky, who control a large share of the Ukrainian economy.
They have not been politically neutered like the oligarchs in Vladimir Putin's Russia, and it's striking that the television stations they own have been covering the demonstrations quite objectively.
The ultimate loyalty of the oligarchs is to their money, of course, but they seem to believe that in the long run their money is safer in EU countries, or at least in a Ukraine that conforms to EU legal standards.
So they are not ecstatic about Mr Yanukovych's decision to turn away from the EU, and they are quite capable of turning away from him. Indeed, that's exactly what they did during the Orange Revolution of 2004, and they could do it again.
So Mr Yanukovych's back is to the wall, and he has apparently decided that it's worth gambling that he can clear the streets by force without triggering a confrontation that spreads far beyond the Maidan. And it will have to be done by force, because the protesters will not just fold their tents and creep off home.
The sudden lurch into violence on the streets on Sunday and Monday nights occurred in this context.
The several hundred young men who attacked the riot police with pipes, chains and firebombs were originally thought to be ''provocateurs'' hired by the Government to give it a justification for using violence on the mass of peaceful protesters, but lots of them were not.
The core group of fighters were members of a radical ultra-nationalist group called Right Sector that is both anti-Russian and anti-EU.
It includes both Russian and Ukrainian speakers, and imagines it can use the current crisis to ''destroy the skeleton state'' and build a new state on the ruins. Things are indeed spinning out of control.
When Vitali Klitschko arrived on the scene to beg them to remain non-violent, he was attacked with a fire extinguisher - and thousands of ordinary protesters showed up to cheer the young thugs as they attacked the police. There is a serious potential for mass violence here, and that could lead to even worse things.
Viktor Yanukovych, for all his faults, is the legitimately elected president of Ukraine, and he has a majority in Parliament. What if, facing overthrow in the streets, he called for ''fraternal aid'' from Russia to defend democracy in Ukraine?
What if the Russians, who are already claiming that it's a Western plot - ''we have information that much of this is being stimulated from abroad,'' Russian Foreign Minister Sergei Lavrov said on Monday - agree to send him police and military help?
It sounds far-fetched and it would be extremely stupid, but everybody is busily painting themselves into corners and there is a small but real possibility that it could happen. In which case, welcome to the Second Cold War.
Gwynne Dyer is an independent London journalist.
