Gunfire could be heard from the Greek side of the River Evros at the Turkish border on Saturday, as migrants attempted to cross the waterway to get into Greece and the refugee crisis that intensified with Syria's civil war shifted back onto the European Union's doorstep.

Some people were seen wading through the water, while others used small, inflatable boats and paddled across. Earlier Saturday, Greek police fired tear gas to push back hundreds of stone-throwing migrants who were trying to cross the border from the Turkish state of Edirne.

The developments came after Turkish President Recep Tayyip Erdogan said on Friday that his country's borders with Europe were open, making good on a threat to let refugees into the continent as thousands of migrants gathered at the frontier with Greece.

The vast majority of the migrants were from Afghanistan and most were men, although there were also some families with young children. Iranians, Iraqis, Moroccans and Pakistanis were also gathered.

We are at the <a href="https://twitter.com/hashtag/Turkey?src=hash&ref_src=twsrc%5Etfw">#Turkey</a>-<a href="https://twitter.com/hashtag/Greece?src=hash&ref_src=twsrc%5Etfw">#Greece</a> border - Greek border police have been frequently firing pepper/tear gas to hold people back. It got very crowded, many have spent the night here in freezing temperatures. More refugees and migrants are arriving, hoping to cross into Europe. <a href="https://t.co/b4qdICev3q">pic.twitter.com/b4qdICev3q</a> —@juliahahntv

Greece, which has tense relations with its neighbour Turkey at the best of times and was a primary gateway for hundreds of thousands of asylum seekers from the Middle East and Africa in 2015 and 2016, has reiterated it will keep this round of migrants out.

Greece said it was sending police and army reinforcements to its land border and reinforcing controls along the sea border, where 52 coast guard and navy vessels were patrolling.

Migrants gather at Turkey's Pazarkule border crossing with Greece's Kastanies, in Edirne, Turkey, on Saturday. (Huseyin Aldemir/Reuters)

"The government will do whatever it takes to protect its borders," government spokesperson Stelios Petsas told reporters.

Ankara said on Thursday it will no longer contain hundreds of thousands of asylum seekers living in its refugee camps after an airstrike on war-ravaged Idlib in Syria killed 33 Turkish soldiers.

Almost immediately, convoys of people appeared heading toward the Greek land and sea borders.

Migrants gather at Turkey's Pazarkule border crossing with Greece's Kastanies, in Edirne. (Huseyin Aldemir/Reuters)

"This has nothing to do with Idlib," Petsas said, adding that in the past 24 hours Greek authorities had prevented attempts by 4,000 people to cross the border.

Greece's Skai TV aired live video from the Turkish side of the northern land border at Kastanies showing Greek riot police firing tear gas rounds at groups of migrants who were hurling stones and shouting obscenities.

A Reuters witness said there were about 500 people in the buffer zone between the two border posts, and beyond that on the Turkish side hundreds more.

Overnight, demonstrators hurled flaming pieces of wood at police, amateur footage filmed by a police official on the scene, which was seen by Reuters, showed.

An estimated 3,000 people had gathered on the Turkish side of the border at Kastanies, a Greek government official said. Kastanies is just over 900 kilometres north-east of Athens.

Migrants are seen during clashes with Greek police at Turkey's Pazarkule border crossing. (Huseyin Aldemir/Reuters)

Without providing supporting evidence, Erdogan said on Saturday that some 18,000 migrants crossed borders from Turkey into Europe after his country "opened the doors" on Friday.

Greek police were keeping media about a kilometre away from the Kastanies border crossing, but the broader area, where the two countries are divided by the Evros River, was more permeable. A group of Afghans with young children waded across fast-moving waters of the river and took refuge in a small chapel. They crossed into Greece on Friday morning.

"Today is good," said Shir Agha, 30, in broken English. "Before, Erdogan people, police problem," he said. Their shoes were caked in mud. It had rained heavily the night before, and by early morning, temperatures were close to freezing.

Greece had already said on Thursday it would tighten border controls to prevent coronavirus reaching its Aegean islands, where thousands of migrants are living in poor conditions.

Nearly a million refugees and migrants crossed from Turkey to Greece's islands in 2015, setting off a crisis over immigration in Europe, but that route all but closed after the European Union and Ankara agreed to stop the flow in March 2016.