Officials with Greece's three main creditors, known collectively as the troika, were under heavy security Tuesday in Athens, with about 250 police officers standing guard outside their hotel, the Telegraph reported, citing Greece’s Mega TV. Technical experts from the three entities began holding talks Monday with Greece on further terms for the latest bailout agreement.

"Teams from the institutions are already on the ground in Athens and work is starting immediately, as we speak,” said Mina Andreeva, a spokeswoman for the European Commission said. "Work has started, meaning that the institutions are talking to the Greek authorities."

The so-called troika -- composed of the European Commission, the European Central Bank and the International Monetary Fund -- were seeking a third round of reforms by the Greek government before giving Greece up to 86 billion euros ($95 billion) as part of a three-year loan to help the country remain in the eurozone.

About six months earlier, the same organizations were expelled from Greece after the left-wing Syriza party came to power and its leaders swore they would not cooperate with the troika and its demands for austerity in Greece as a condition for billions of euros in bailout cash.

The troika has become vastly unpopular with Greeks, who have angrily protested the creditors’ presence in the past. In 2013, a man threw coins at Poul Thomsen, then the mission chief for the IMF. Thomsen has since retained the services of a bodyguard, 24 hours a day, when he is in Athens, Bloomberg reported.

In April 2014, one day before German Chancellor Angela Merkel, who has championed austerity for Greece, was due to visit the capital, a car bomb exploded outside the Bank of Greece building in Athens. No one was injured or killed. Thousands of protesters also took to the streets ahead of her visit.

Negotiations that began this week were reportedly delayed because the Greek government sought to limit the mobility of troika members in Greece’s capital, Bloomberg reported. When they visit Athens, members work out of a heavily secured and bulletproof office bearing no identifying markers.

Greece’s stock market could reopen as soon as Wednesday, pending a decision by the European Central Bank. The stock exchange has been closed since June 29, Reuters reported.