The capital cities of Greece and Bulgaria witnessed demonstrations Thursday as hundreds poured out on to the streets to protest Turkey shooting down a Russian warplane earlier this week, according to reports.

Demonstrators in Athens burned Turkish and U.S. flags, and demanded that Greece pull out of NATO. Protesters gathered at Syntagma Square and marched toward the Turkish embassy but were stopped by police, Ria Novosti news agency reported. An additional police force was deployed at the embassy.

Turkey downed a Russian Su-24M fighter jet Tuesday, accusing Russia of violating its airspace despite repeated warnings.

Panagiotis Lafazanis, leader of Greece’s left-wing party Popular Unity, said that Turkey’s actions were “unjustified” and a “clear act of war,” RT.com reported.

The protesters also demanded an official denouncement of Ankara’s move, and voiced support for Russia’s air campaign in Syria. Russia has intensified its airstrikes against the Islamic State group after ISIS claimed responsibility for shooting down a Russian passenger jet over Egypt's Sinai Peninsula last month, killing 224 people.

In Bulgaria, people demonstrated in the capital Sofia and the Ataka party’s leader Volen Siderov praised the Russian operation in Syria, adding that the U.S. had not dealt with ISIS efficiently.

“We are under a situation in which Russia is fighting against terrorism. And effectively. For two months, the Russian army has done everything that the Americans, French and German people and others couldn’t do for a year and a half,” Siderov said, according to RT.com.

Russian President Vladimir Putin has called the downing of the fighter jet a “stab in the back” and said Turkey has sided with terrorists. Russia also began toughening up controls on Turkish food imports and, on Thursday, Russian Prime Minister Dmitry Medvedev announced that government authorities were given two days to make a list of ways to curb economic ties and joint projects with Turkey.