European Union foreign policy chief Catherine Ashton said on Tuesday she was "appalled" by the execution of 10 Iranians convicted of drug trafficking offences.
Speaking amid controversy over a planned European parliamentary mission to Iran from October 27 to November 2, Ashton said in a statement that Monday's executions add "to the alarming execution rate in Iran" now at over 300 since the beginning of the year.
"Most of the executions took place after summary trials, without the right to appeal and for offences which according to international minimum standards should not result in capital punishment," she added.
"I call on Iran, once more, to halt pending executions and to introduce a moratorium on the death penalty.
The 10 were hanged in a Tehran prison despite appeals from rights groups to stop the executions.
MEPs from all but two of the parliament's groups -- the conservative EPP and eurosceptic right-wing ECR alliance -- are planning to go on the visit to Iran, originally agreed in July, an official said last week.
The team will be headed by the president of the European Parliament-Iran group, Finland Greens MEP Tarja Cronberg.
The official said there was a "sharp" discussion on whether to go ahead with the visit but that leaders of the parliament's political groups finally had voted in favour.
The European Union last week imposed a tough package of sanctions against Iran, aiming to push global talks with Tehran on its contested nuclear programme.
Both the parliament and the bloc are constant critics of the country's human rights record.