Saakashvili, 50, was thrown out of Ukraine in mid-February when masked men abducted him from a downtown Kyiv restaurant and swiftly deported him to Poland.
He is currently living in the Netherlands with his Dutch wife and two sons.
Now his lawyers say that Kyiv trampled on the rights of the maverick politician, but any lawsuit – for instance at the Luxembourg-based European Court for Human Rights – could take many years to pursue.
Instead, Saakashvili called upon European leaders to slap sanctions on Poroshenko and his political allies to force the Ukrainian president to allow him back into the country.
Ukraine is due to hold parliamentary and presidential polls next year, which Saakashvili has vowed to contest.
“I will return to the Ukraine, but in the meantime I’ll keep busy to ensure that special sanctions are being put into effect against Poroshenko,” Saakashvili told reporters at his first press conference since arriving in the Netherlands in February.
His British lawyer Geoffrey Robertson said his client decided to call on European leaders to impose sanctions against Poroshenko because a possible lawsuit “would still take years”.
“The challenge, I think, is now to the states of Europe ... to take diplomatic action ... that must be used as a lever to actually influence Ukraine and its president to do the right thing,” Robertson said.
“To allow Mr Saakashvili back, to allow him to lead his political party in the elections next year,” Robertson said.
A one-time regional governor in Ukraine, Saakashvili dramatically fell out with Poroshenko over allegations that Kyiv was failing to fight endemic corruption.
The ugly dispute saw Saakashvili stripped of his Ukrainian passport, effectively rendering the former leader stateless.
He has now received a Dutch ID card which allows him to live and work in the European Union.
Kyiv in turn accuses Saakashvili of trying to stage a coup sponsored by allies of former Kremlin-backed Ukranian president Viktor Yanukovych – a charge he strongly denies.
Saakashvili was detained at the Kyiv restaurant and flown back to Poland, after slipping into Ukraine through a blockade on the Polish-Ukrainian border in September 2017.
The former Georgian president from 2004-2013 is also wanted by Tbilisi on charges of abuse of power – which he strenuously denies.
A Georgian court sentenced him to three years in prison in absentia in January.
