Refugees can now gain access to Europe through Turkey.
Turkey shares borders with two European Union countries, Greece and Bulgaria.
Turkey's security forces have been ordered not to stop the exodus, at least temporarily reversing an arrangement made with the EU in 2016.
The decision comes after 33 Turkish soldiers were killed by Syrian government forces in northern Syria's Idlib region Thursday.
About 1 million Syrian refugees have been displaced and have gathered near the border with Turkey since December.
Turkey already houses nearly 4 million Syrian refugees.
Early Friday, close to 300 migrants began the trek to Turkey's Edirne province on the Greek border.
Media reports said that in addition to the Syrians making the trip, Iranians, Iraqis, Pakistanis and Moroccans were also part of the group.
Refugees are also heading toward Ayvacik, where they hope to travel by boat to the Greek island of Lesbos.