The Latest: S&P cuts Greek credit rating amid turmoil

Standard & Poor's rating agency has cut Greece's credit grade by one notch further into junk status amid the country's deteriorating financial situation.

The agency says it now sees a 50-percent chance of Greece leaving the eurozone.

In a statement, S&P says it interprets the Greek government's decision to hold a referendum on the rescue creditors' proposals for a bailout deal as indication that it "will prioritize domestic politics over financial and economic stability, commercial debt payments and eurozone membership.

Meanwhile, waving Greek flags and chanting "take the bailout and go!" thousands of pro-government protesters gathered outside parliament late Monday to back Prime Minister Alexis Tsipras' call to oppose a proposed compromise with creditors in a referendum.

Police said 13,000 people attended the rally in Athens' main Syntagma — the largest referendum rally so far— in a generally good natured protest joined by casually-dressed cabinet ministers.

"The government tried too hard to get this agreement. But the creditors kept asking for more.

I don't know what else we are supposed to do," said pensioner Satroula Noutsou, who joined a rowdy group of protesters, singing slogans as one demonstrator banged a drum.

"Tsipras is one of us, a regular person. And there are some people who don't like that."

And European Union President Donald Tusk has warned the Greek people that a "no" vote in the referendum won't give their government more leverage to seek a better deal with Greece's creditors.

Tusk said a news conference in Brussels that "every government has a right to hold a referendum, therefore we respect the Greek decision."

"However, one thing should be very clear: if someone says that the government will have a stronger negotiating position with the 'no' vote, it is simply not true."

He added: "I'm afraid that which such a result of referendum, there will be even less space for negotiation."