Turkey and Russia, who back opposing forces in the Syria conflict, have held talks to defuse tensions after an air strike killed the Turkish troops, sparking fears of a broader war and a new migration crisis for Europe.
But Erdogan raised the stakes on Saturday and vowed to allow refugees to travel to Europe from NATO-member Turkey as a way to pressure EU governments over the Syrian conflict.
Turkey already hosts 3.6 million Syrian refugees.
Erdogan's comments were his first after 34 Turkish troops were killed since Thursday in northern Syria's Idlib where Moscow-backed Syrian regime forces are battling to retake the last rebel enclave.
"What did we do yesterday? We opened the doors," Erdogan said in Istanbul. "We will not close those doors.... Why? Because the European Union should keep its promises."
He was referring to a 2016 deal with the European Union to stop refugee flows in exchange for billions of euros in aid.
Erdogan said 18,000 migrants have amassed on the Turkish borders with Europe since Friday, but that could reach as many as 30,000 by Saturday.
There were skirmishes on the Turkish-Greek border at Pazarkule on Saturday with Greek police firing tear gas to push back thousands of migrants, according to AFP photographer in the western province of Edirne.
Ahmad Barhoum, a Syrian refugee, said he had been trapped at the border since Friday.
"If they do not open we will try to cross by illegal means," he told AFP.
"I hope that they will end up letting us in so that we can start a new life in Europe worthy of human beings," said one Egyptian refugee.
In 2015, Greece became the main EU entry point for one million migrants, most of them refugees fleeing the Syrian civil war.
"A situation like the one in 2015 must not be repeated," Austrian Chancellor Sebastian Kurz said on Twitter. "Our aim must be to adequately protect the EU's external borders, to stop illegal migrants there."
In Athens, Greek Prime Minister Kyriakos Mitsotakis held an emergency meeting to discuss the border.
"We averted more than 4,000 attempts of illegal entrance to our land borders," government spokesman Stelios Petsas said after the emergency meeting.
A Greek police source said migrants had set fires and opened holes in border fences.
Police and soldiers patrolled the Evros river shores - a common crossing point - and warned with loudspeakers not to enter Greek territory.
Criticising Greece's reaction, Turkish Foreign Minister Mevlut Cavusoglu tweeted: "Look who's lecturing us on international law! They're shamelessly throwing tear gas bombs on thousands of innocents piled at their gates. We don't have an obligation to stop people leaving our country but Greece has the duty to treat them as human beings!"
According to Hellenic Coast Guard, from early Friday to early Saturday 180 migrants reached the islands of Eastern Aegean, Lesbos and Samos in sea crossings.
One rubber dinghy arrived early Saturday in Lesbos carrying 27 African migrants from Turkey. Many of them were women, who wept and prayed on their knees, according to an AFP reporter.
The UN said nearly a million people - half of them children - have been displaced in the bitter cold by the fighting in northwest Syria since December.
Turkey said its forces destroyed a "chemical warfare facility," just south of Aleppo, in part of its military retaliation after its soldiers were killed.
"We would not want things to reach this point but as they force us to do this, they will pay a price," Erdogan said.
Syria's state media denied the attack and the existence of such a facility.
Thirty-three Turkish soldiers were killed in an air strike by Russian-backed Syrian regime forces on Thursday. A 34th has since died.
The latest incident has raised further tensions between Ankara and Moscow, whose relationship has been tested by violations of a 2018 deal to prevent a regime offensive on Idlib.
As part of the agreement, Ankara set up 12 observation posts in the province but Syrian President Bashar al-Assad's forces - backed by Russian air power - have pressed on with a campaign to take back the territory.
On Friday, Erdogan spoke by phone with his Russian counterpart, Vladimir Putin, in a bid to reduce tensions.
Erdogan may travel next week to Moscow for talks, according to the Kremlin.
But the Turkish leader remained critical on Saturday.
"I asked Mr Putin: 'What's your business there?'," Erdogan said. "If you establish a base, do so but get out of our way and leave us face to face with the regime."