The delegation of a dozen members of the Party of European Socialists (PES) sought to make a visit to the head of the Peoples' Democratic Party (HDP) Selahattin Demirtas at the prison in Edirne in northwest Turkey.
But they were blocked by Turkish gendarmes on the approach road to the prison and instead held an impromptu press conference in the street, an AFP correspondent said.
Demirtas' lawyers were however allowed to visit him.
He is one of 10 HDP MPs held, including his co-leader Figen Yuksekdag, following their arrest earlier this month.
They are accused of membership of or promoting the outlawed Kurdistan Workers Party (PKK) and are under arrest in various prisons across the country pending trial.
"He is not abandoned, he is not alone, our political family is in solidarity with him," said PES president and Bulgarian MEP Sergei Stanishev who was joined by MEPs and national lawmakers from Sweden, France and Austria.
The former Bulgarian premier added that there can be "no compromise" on political criteria, as Turkey seeks to join the European Union in a long-stalled bid.
Hisyar Ozsoy, the vice chairman of the HDP in charge of international affairs, slammed the reluctance of the authorities to allow visits to Demirtas.
"There is a serious policy of isolation ... they have reduced relations with the outside world to the lowest level," he said.
According to the HDP, over 50 pro-Kurdish mayors have also been detained in the crackdown, the latest being Mardin mayor Ahmet Turk who was taken in on Monday.
Critics complain the authorities are using the state of emergency imposed in the wake of the July 15 coup attempt to crack down on all opposition, not just the suspected coup plotters themselves.