Maasoum Marzouk was arrested on Thursday after calling for a referendum on President Abdel Fattah el-Sisi’s rule.

An opposition coalition of left-leaning and secular parties in Egypt has called for the immediate release of a former diplomat who was arrested after calling for a referendum on President Abdel Fattah el-Sisi‘s government.

Maasoum Marzouk was arrested on Thursday along with two others and ordered to remain in custody for 15 days pending an investigation into terror-related charges.

Marzouk and the other detainees – economist Raied Salama and Yehia al-Qazzaz, a geology professor – were accused by state-run media of cooperating with the banned Muslim Brotherhood group, which Egypt’s government designated as a “terrorist” group in 2013.

On Monday, the Civil Democratic Movement coalition said in a press conference that the arrests were “part of the government crackdown to silence opposition voices”.

At the press conference, Mohammed Anwar Sadat, nephew of Egypt’s former leader Anwar Sadat, called for “genuine political reform” and “for the President and the state to listen” to the opposition.

Marzouk, who was assistant minister of foreign affairs under former President Hosni Mubarak, served in the Egyptian army’s special forces and is a veteran of the 1973 war with Israel.

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He is also a former Egyptian ambassador to Uganda, Finland and Estonia.

More recently, he helped to form the The People’s Democratic Party and served as an adviser and spokesman for the presidential campaign of Hamdeen Sabbahi, a left-wing candidate who ran against Sisi in the 2014 elections.

Speaking at Monday’s press conference, Sabbahi called for change, saying Sisi’s government had failed.

“We believe that this regime must be changed,” he said. “This is a failed power. This is a repressive power … which violated and insulted the constitution.”

In a statement on August 5, Marzouk called for protests in Tahrir Square in the Egyptian capital, Cairo, saying that if his initiative was not accepted, he would hold a “popular conference” there.

The Square was the epicentre of the 2011 mass uprising that led to the downfall of Mubarak.

He had also campaigned for the release of all political prisoners, the formation of a transitional government council and a 10-year ban on political candidacy for anyone who had served in government or parliament over the past decade.

Sisi won a second four-year term in April after securing more than 90 percent of the vote in a presidential election that was criticised as being a one-man show with no credible opposition.

At least six candidates were either jailed, prosecuted or forced to pull out of the race.

Since coming to power after leading the overthrow of Mohamed Morsi in 2013, Sisi’s government has presided over a sweeping crackdown on dissent, jailing thousands of dissidents and journalists.

The government has banned unauthorised protests, heavily restricted civil society groups and blocked hundreds of websites.