Alexis Tsipras said negotiations were at a "critical" stage, but that the lenders' proposals were "not realistic".
He was briefing parliament amid growing opposition in his leftist Syriza party to the creditors' proposals.
Earlier Greece delayed Friday's €300m (£216m) debt repayment to the IMF.
Mr Tsipras described the EU-IMF lenders' plan as a "bad moment for Europe" and a "bad negotiating trick".
He accused Greece's lenders of massively backtracking on measures agreed in recent months, and of failing to see the need for an end to austerity in their latest offer.
A call for debt relief was a key part of the Greek plan. » | Friday, June 05, 2015
