Turkish President Recep Tayyip Erdogan has said troops have begun moving into Libya after parliament approved the move last week.
He said their mission was to ensure stability for the UN-backed Government of National Accord (GNA) in Tripoli.
The Libyan government is fighting an insurgency by rebel forces under Gen Khalifa Haftar, based in eastern Libya.
Gen Haftar is backed by Egypt and the UAE, while the UN-backed government is supported by Turkey and its ally Qatar.
Rebel forces have been trying to capture Tripoli and were blamed for an air strike on a military academy on Saturday that killed at least 30 people. They denied any involvement.
“Our soldiers’ duty there is co-ordination. They will develop the operation centre there. Our soldiers are gradually going right now,” President Erdogan told the CNN Turk TV channel.
He said Turkey’s objective was “not to fight” but “to support the legitimate government and avoid a humanitarian tragedy”.
Israel, Greece and Cyprus had warned against any Turkish deployment. They called it a dangerous threat to regional stability, and warned that it breached a UN arms embargo.
The UN Security Council is expected to meet behind closed doors on Monday to discuss the situation in Libya, AFP news agency reported, citing diplomats.
MPs in Turkey approved the bill allowing the deployment of troops last Thursday with 325 in favour and 184 against.
Mr Erdogan said Tripoli had requested military assistance.
The Turkish government has given no details about the scale of the military deployment.
Libya has been torn by violence since long-time ruler Muammar Gaddafi was deposed and killed in 2011 by Nato-backed forces.
The country has two rival administrations, one based in Tripoli and the other in the eastern city of Tobruk.