A Greek athlete has been expelled from the Olympics after failing a drugs test, the country's team has said.
It said the unnamed athlete's positive result came from a test carried out in July in Athens.
The barred contestant has left the Olympic village, they added.
The Games officially start later on Friday.
A boat carrying hundreds of refugees and migrants has capsized off the Greek coast, sparking a major rescue operation, Greek authorities have said.
According to The Associated Press, more than 300 people have been lifted from the semi-sunken boat in the Mediterranean, which overturned around 75 nautical miles south of Crete in international waters, while three bodies have been recovered.
It comes after the Italian navy rescued hundreds of migrants from a boat which capsized off the coast of Libya yesterday.
BREAKING: Greek coast guard says 302 people rescued, 3 bodies recovered in migrant boat rescue operation.
A spokeswoman for the Greek coastguard said hundreds more could still be in the water.
"People are in the water, boats crossing the area have thrown lifebuoys and are moving to save the migrants," she added.
It was not immediately clear where the boat, described as a fishing-style vessel of around 30 metres (98ft) in length, had come from, where it was headed, or where its passengers had originated.
Two Greek patrol boats and two helicopters have been sent to the scene, while four ships which had been sailing nearby were also taking part in the rescue operation.
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At least 300 migrants have been injured after violent clashes with authorities at the Macedonian border with Greece, charity Médecins Sans Frontières (MSF) said.
Authorities in Macedonia have been accused of using "dangerous and deplorable" force against migrants protesting at the Greece's border town Idomeni.
"The situation was extremely tense this morning, when tear gas, rubber bullets and stun grenades were used in the camp to disperse crowds. The MSF teams treated 300 people, among them 200 with respiratory problems after being subjected to teargas," the charity said in a statement.
Police sources say tear gas was fired only after migrants stormed towards a border fence on Sunday.
The EU is now deporting new arrivals to Greece back to Turkey if they don't meet asylum criteria after agreeing a deal with the country.
A second boat has left the Greek island of Lesbos taking 79 more migrants to Turkey as part of a deal to return those crossing the Aegean Sea to enter Europe.
One woman was among those on board the Turkish ferry, which left Mytilene port in Lesbos.
Another Turkish ferry carrying 45 migrants left the port earlier.
The migrants sent to Turkey today were mainly from Pakistan.
Forty-five migrants were taken by ferry from Lesbos to Turkey as Greek authorities restarted returns under an EU deal to stem the flow of migrants and refugees to Europe.
The ferry left the port of Mytilene on Friday morning, with a second boat expected to leave later, state television reported.
Two protesters at the port jumped into the sea and began swimming towards the boat and were pulled out of the water by the Greek coastguard.
The first group of 202 migrants to be returned, most of them from Pakistan and Afghanistan, were sent back to Turkey on Monday.
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The EU-Turkey agreement to tackle the migrant crisis took effect on Monday but Greek authorities have warned the operation is threatened by a shortage of personnel.
A Downing Street spokesman said Mr Cameron spoke to Mr Tsipras on the phone on Tuesday afternoon.
The Prime Minister welcomed the start the Greek authorities had made to returning migrants to Turkey under the agreement, and reiterated the importance of breaking the link between people making the dangerous crossing of the Mediterranean and securing settlement in Europe... The Prime Minister confirmed that additional UK personnel would be offered to Greece over the coming weeks to provide additional support.