Police tried to storm a camp set up by Saakashvili supporters in front of parliament. Officers were seeking to detain the Georgian maverick politician, 49, whose supporters had helped him flee police custody on Tuesday.
Saakashvili's supporters, armed with sticks, axes and chainsaws, fought back, police said. Fourteen officers and two civilians were injured, some with broken bones and head injuries.
Saakashvili, a former Ukrainian governor, fell out with Poroshenko and was stripped of his Ukrainian passport. He forced his way back into the country with the help of supporters in September and since then has led rallies calling for Poroshenko's ouster.
"They were looking for me but they didn't find me," Saakashvili told journalists in the camp on Wednesday, speaking beside his wife Sandra Roelofs.
"Here there are really just peaceful people. There aren't any weapons," he said, calling for Ukrainians to demonstrate in front of parliament in the evening.
Anti-corruption drive
Kiev has accused Saakashvili of attempting to take power by force through recent anti-corruption protests.
Prosecutors say the demos were financed by associates of Ukraine's former Moscow-backed president Viktor Yanukovych, who was toppled in 2014 by pro-European protests and fled to Russia.
The former Georgian leader has called the claims a "provocation".
Saakashvili presents himself as an anti-corruption crusader in Ukraine but has only limited public support. He rejects Kiev's accusations and in turn accuses Poroshenko of orchestrating a campaign against him.
During a raid by Ukrainian security authorities at his home on Tuesday morning, Saakashvili defiantly climbed onto the roof of his apartment building and gave a speech to supporters in the street below.
He was detained and driven away in a police van but hundreds of his supporters then swarmed around the vehicle and freed him.
Saakashvili is best known for reforms aimed at reducing Georgia's ingrained corruption after gaining power during the so-called Rose Revolution in 2003.
He is an arch-enemy of Russia's President Vladimir Putin and while he was president of Georgia the countries fought a brief war in 2008.
He moved to Ukraine in 2015 in the wake of a pro-Western revolution there, acquired Ukrainian nationality and served as governor of the key Odessa region on the Black Sea.
But he later dramatically fell out with Poroshenko after suggesting Kiev was flagging in its fight against corruption.
He was thrown out of the country but made a dramatic return in September, forcing his way through the border and promising to continue his fight against oligarchs he says are destroying the economy.
Around 1,000 supporters of Saakashvili rallied Wednesday in the Georgian capital Tbilisi to protest against his attempted arrest in Ukraine.
Waving Georgian and Ukrainian national flags, protesters marched through downtown Tbilisi, briefly blocking traffic on the city's main thoroughfare.
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