Athens says it will push back any undocumented migrants from Turkey as Ankara opens borders for Europe-bound refugees.

Greece is bracing itself for what could turn into a flood of refugees and migrants after the Turkish government ordered its coastguard and border police not to prevent people from crossing into Europe.

“No illegal entries into Greece will be tolerated,” Prime Minister Kyriakos Mitsotakis tweeted on Friday after Greek police fired tear gas at about 300 refugees trying to cross the land border at the Evros river.

On Saturday, a Greek government spokesperson claimed to have “averted more than 4,000 attempts of illegal entrance to our borders.”

Later on Saturday, Turkish President Recep Tayyip Erdogan said 18,000 refugees and migrants had gathered on the Turkish borders with Europe since Friday, adding that the number could reach as many as 30,000 on Saturday.

Meanwhile, tensions are exploding at the Greek border, with riot police firing tear gas at groups of arriving refugees, some of whom are allegedly throwing stones and pieces of flaming wood in protest.

Greece’s land border with Turkey is relatively strong. It is160km (99 miles) long and contains natural defences such as the Evros river and its marshy delta. Greece reinforced it with extra patrols and thermal cameras in recent months.

The maritime border is another story. Hundreds of kilometres long, it is patrolled by about 40 Greek coastal patrol vessels and boats, aided by a European Border and Coast Guard force.

Greece is in the process of building another 19 vessels but on Friday asked the European Union to provide more assistance.

It is physically difficult to intercept refugees at sea and Greece’s archipelago presents them with thousands of islands to alight upon.

Since 2015, when a million refugees crossed the Aegean into Europe, Greece has found that the only real defence is diplomacy – persuading Turkey to put its coastguard vessels back into action to pick up refugees before they reach the Greek-Turkish territorial waterline.

Turkey opened the borders after dozens of its soldiers were killed in an air raid in Idlib, Syria, and has since complained that it lacks international support for its military campaign and that it hosts the world’s largest number of refugees.

Mitsotakis on Friday said: “Greece does not bear any responsibility for the tragic events in Syria and will not suffer the consequences of decisions taken by others,” he wrote – a reference to Turkey’s military support for groups opposed to the Syrian government.

Greece also has expectations of Europe. With 1 percent of Europe’s GDP and 2 percent of its population, it finds itself processing almost 11 percent of EU asylum applications – a result of rules requiring asylum seekers to apply in the country they arrived in. So far, the EU has been unable to negotiate a permanent burden-sharing mechanism.

Some Greek officials also see the EU’s deeper involvement in the Middle East as a prerequisite to resolving its refugee woes.

“Europe has to decide what to do, because no matter how many people you resettle or save at sea, more people will come,” a senior security official in the Greek government told Al Jazeera on condition of anonymity.

“Europe has to make serious decisions but right now its politicians seem to have a [particular] narrative that prevents them from doing so, while societies are suffering from information overload and an economic crisis.”

Arrivals on the islands were not unusual on Friday – 151 asylum-seekers on five boats. But that could change very quickly.

Mercantile Marine Minister Ioannis Plakiotakis was on Lesbos in a largely symbolic visit to demonstrate solidarity with the east Aegean.

“We are here with the chief of the coastguard to emphasise our resolve to protect our maritime borders to the highest degree possible,” he told Al Jazeera.

Right-wing government’s refugee policy

The eight-month-old right-wing New Democracy government has been frustrated in implementing its harder refugee policy.

On January 1, it started implementing a stricter asylum law that aims to speed up processing and increase returns to Turkey but the results have not been spectacular. Returns so far are in the dozens per month.

The government has also failed to persuade the five islands with reception centres – Lesbos, Samos, Chios, Leros and Kos – to allow it to build detention centres that will replace current open camps, increasing total capacity on the islands.

Despite the fact that camps on the five islands are currently overflowing with 42,000 asylum-seekers, islanders say that increasing official capacity will lift numbers even further.

In recent days, riot police ferried in from Athens clashed with locals on Lesbos and Chios as construction companies attempted to bring in earth-moving machinery to start work on the new camps.

“The refugee issue is of national importance,” said Stefanis. “Everyone must help above and beyond political interests and affiliations.