Turkish opposition officials have warned of a campaign of harassment and intimidation by the government in the run-up to next month’s referendum on a presidential system that would grant sweeping powers to President Recep Tayyip Erdoğan.

A wave of arrests of opposition lawmakers, activists and journalists, and the closure of media outlets, have left a predominantly government-friendly press moderating the debate on the vote. .

Opposition figures have also highlighted the divisive rhetoric of the ruling AK party, whose officials have conflated opposition to the constitutional package with support for terror groups like Islamic State. The vote will be held under the state of emergency imposed after a failed coup attempt last July.

“In democracies people can pick and choose what they want,” said Barış Yarkadaş, a politician in the opposition People’s Republican Party (CHP), which is backing a no vote. “Now, the citizens’ right to pick and choose is being hijacked because the citizens can only hear one voice, and that is ‘yes’.”

Erdoğan approved the constitutional amendments last month after they were passed by a simple majority in parliament, paving the way for the vote on 16 April..

The referendum is widely seen as a vote on Erdoğan’s leadership. If accepted, the changes will allow the president to stand for two more terms, potentially keeping him in power until 2029.

A tank on the streets of Istanbul during the failed coup in 2016. Photograph: Adem Altan/AFP/Getty Images

The constitutional changes also include increasing the number of parliamentarians and reducing their age, abolishing the role of prime minister, allowing the president to create a cabinet that will have little oversight by parliament, and giving both the president and parliament the ability to call early elections.

The president’s supporters say the changes will lead to a strong Turkey no longer subjected to the chaos of coalition governments, will resolve conflicts of power in the executive branch, and will establish checks on the president’s power with the ability to impeach him or call new presidential elections.

But critics fear Turkey becoming a country under one-man rule, with power concentrated in the hands of Erdoğan, who will consolidate his authority over a friendly parliament and judiciary.

They also complain of oppression on a grand scale in a country reeling from a series of terror attacks, as well as the July coup attempt and an ensuing purge of the civil service.

A dozen opposition politicians from the pro-Kurdish Peoples’ Democratic party (HDP) are currently in prison, and last week judges handed down punishments to the two co-chiefs of a leftist coalition that includes Kurdish politicians and activists as well as other minority groups.

The concept of plurality is being abolished Barış Yarkadaş

Selahattin Demirtaş, the charismatic Kurdish politician who has spoken out against a presidential system and has been incarcerated since November, was sentenced to a further five months in prison. His co-chair Figen Yüksekdağ was stripped of her parliamentary seat after a conviction by the high court for her attendance several years ago at the funeral of a leftist militant. The HDP said the ruling was “an attempt to intimidate us and our people”.

“The only leader who can maintain a good no campaign against Erdoğan is Selahettin Demirtaş,” said Mithat Sancar, an HDP MP. “Demirtas’s arrest means Erdoğan has pushed his most powerful rival out of the game.”

Sancar said a referendum under emergency law would inevitably lead to more division in a country that is already politically polarised.

Senior officials from the ruling AK party have appeared to link voting no with promoting the interests of terror groups like Isis, the PKK and the Gülenists, followers of a US-based exiled cleric accused of responsibility for the attempted putsch.

Prime minister Binali Yıldırım has said terror groups are opposed to the constitutional changes. “If all terror organisations are carrying out no campaigning like a chorus, then that should have meaning for our country, our people and our citizens. My citizens will not be on the same side as terrorists.”

Ozan Erdem, a provincial AKP deputy, was forced to resign by his own party last month after saying the country would have to prepare for civil war if the constitutional changes were defeated in the vote.

Protesters outside the offices of the Cumhuriyet newspaper demand the release of journalists. Photograph: Yasin Akgul/AFP/Getty Images

Beyond the rhetoric, the opposition says they have few outlets to express their views. The CHP estimates that 152 journalists are currently in prison, including 11 from Cumhuriyet, the country’s oldest newspaper, which has taken a tough stance against the reforms and has been hounded by lawsuits and the threat of having a government-appointed trustee board take over management. More than 170 media organisations have been shut down since the coup, including newspapers, websites, TV stations and news agencies, and 2,500 journalists have been laid off.

This week, German-Turkish journalist Deniz Yücel of Die Welt newspaper was formally arrested after reporting on the hacking and leak of the personal emails of Erdoğan’s son-in-law, in a move interpreted as a message to the foreign press in the country.

Nearly 5,000 academics have been dismissed, part of a purge of professions including the police, army and judiciary in which more than 125,000 people have been fired and 40,000 arrested following the coup attempt.

“The concept of plurality is being abolished … and Turkey is being put in a position where there is only one voice,” said Yarkadas, the CHP MP who heads his party’s media commission and recently met the imprisoned Cumhuriyet journalists.

Some observers see the government’s intimidation of the no campaign as a sign that it fears a divided electorate may be leaning towards rejecting the constitutional changes. Polls have varied wildly, though some internal surveys show the opposition with a lead. Key to the result will be swing voters, who make up an estimated 10% of the electorate.

Few expect a fair fight. The government has removed the Supreme Election Board’s authority to impose penalties on TV stations that fail to give sufficient air time to both campaigns. As one media official, who asked to remain anonymous, put it: “Turkey debates in whispers right now.”

Gülsin Harman contributed reporting