After more than a month of protests, Bulgaria’s longstanding prime minister, Boyko Borissov, said last week he would resign if parliament approved his call for the election of a grand national assembly to “restart the state”.

In a televised address, Borissov said his party would propose a list of amendments to the constitution, including aimed at halving the number of MPs from 240 and reforming the judiciary.

Until recently Borissov had been able to weather most political storms, from secret recordings of him cursing political opponents to leaked photographs of him sleeping with a gun by his bed.

But protests calling for his resignation and that of his cabinet and prosecutor general have drawn tens of thousands of people in the past month and won the support of the president, Rumen Radev.

The unrest began when a former justice minister, Hristo Ivanov, and two other people sailed to a beach near the Black Sea port of Burgas and tried to plant a Bulgarian flag. The beach, supposedly public, is close to a mansion owned by Ahmed Dogan, a former politician from the ethnic Turkish DPS party with shadowy business interests. State security officers manhandled the three men and tossed the flag into the sea.

The incident transformed Ivanov’s image from detached intellectual to maverick politician setting the terms of public debate, and his centre-right Democratic Bulgaria coalition doubled its support in the polls. “This was not simply the PR action of the year but of the decade,” said Petar Cholakov, a political analyst and sociologist from the Bulgarian Academy of Sciences.

The goal of the stunt was to “reclaim” the beach and highlight what Ivanov described as “a government that is literally dependent on Delyan Peevski,” a media mogul and MP also from the DPS party. DPS was founded to protect the rights of the Turkish minority repressed by the former totalitarian regime, but Peevski has allegedly used its kingmaker status in parliament to become a major player in the Bulgarian economy, owning several construction companies and an estimated 80% of Bulgaria’s print media.

Ivanov’s actions resonated with many Bulgarians, sparking a feud between Borissov and the Socialist-backed Radev over the publicly funded protection of Dogan and Peevski. Prosecutors raided Radev’s office and arrested two of his advisers, fuelling public anger.

“We will take back Bulgaria! Thugs out,” said Radev at the end of the first protest, siding with the street against a government and head prosecutor seen by many Bulgarians as deeply corrupt.

Bulgaria, a former Soviet bloc nation of 7 million people, joined the EU in 2007 but remains an economic laggard plagued by high-level corruption. Ivanov said the public was gradually coming to recognise the need for reform of the judiciary and for the prosecutor general to be held accountable.

“I spent 14 years fighting for the public to start understanding what are the problems of the Bulgarian judiciary and I know how difficult it was to raise awareness,” he said.

In his 15-month stint as justice minister in 2014-15, Ivanov embarked on an ambitious mission to reform the judiciary and limit the influence of the prosecutor’s office over the rest of the system. He tried to go after some notorious magistrates, and launched investigations that could have reached the highest offices in the judiciary and executive branches.

His attempt to change the system from within failed quickly. “I knew my time was over when it became clear that there will be no serious [high-level corruption] probes, that the European commission is not going to support my calls for investigation, and at the same time the constitutional amendments I was pushing for were abandoned by the majority in parliament,” he said.

Borissov’s reluctance to wholeheartedly embrace reform did not surprise Ivanov. Borissov has dominated Bulgarian politics for the past 15 years without having strong ideological convictions.

Ivanov compared Borissov to the former Italian prime minister Silvio Berlusconi, saying he had “the same type of corrupting charm, media engagement and [a habit of] exploiting the deficiencies of a particular media landscape”. Borissov has long benefited from a cozy relationship with DPS-linked media.

Ivanov’s previous experience of asking for – and not receiving – backing from the European commission has left him with few illusions about the potential role of the EU in the reform process in the country.

“I understood after my stint as a minister that we have to stop hoping for some force from outside to help us,” he said. Asked if he expected the recently appointed European public prosecutor, Laura Kovesi, to help Bulgaria tackle its corruption problems any time soon, he said: “She has very complex bureaucratic tasks before that and it will take her years.”

Last week Borissov made his tentative offer to step down. “I am ready to leave at any time, if this is what matters,” he said to a gathering of 4,000 party members assembled at short notice. The audience shouted “no” in response, suggesting the offer was probably a stunt.

The ambiguity has only fuelled public dissatisfaction further. Tent camps have blocked main arteries in the capital, Sofia, and across the country, and thousands of people have joined the daily anti-government marches.

According to Cholakov, the longer Borissov remains in power, the more his party will suffer electorally in the next elections. “Entrenching in power at this moment looks like the worst possible decision for the ruling majority,” he said, noting that polls suggested all parliamentary parties – including the socialist opposition – were losing out compared with a month ago.

Ivanov said: “Borissov is resolving the most difficult political equation in the life of a politician: it is not how to make a government, but how to get out. The longer you stay, the more power you concentrate, the more difficult it becomes to let go of the levers of government and survive.”

Additional reporting by Shaun Walker and Jon Henley