The Maldives will hold presidential elections on Sunday amid warnings the Indian Ocean archipelago is reverting back to the authoritarian rule it threw off in a democratic spring a decade ago.

Opposition parties from across Maldivian politics have formed a united front to oust the president, Abdulla Yameen, whose government has been accused of crushing dissent, corruption and jailing electoral opponents on flimsy grounds.

Yameen is presenting himself as a Maldivian nationalist focused on economic development, pointing to infrastructure projects built during his term, including a 2km bridge linking Malé to the international airport that opened earlier this month.

Many of these projects have been funded by an estimated $1.3bn in loans from China, a debt equal to more than one-quarter of the Maldivian GDP, and which western diplomats are concerned will leave the country vulnerable to Chinese influence.

The Sinamale bridge, which connects Malé with Velana international airport, was financed with $126m grant from China. Photograph: -/AFP/Getty Images

Campaigning has been muted in the capital, Malé, with opposition forces permitted to hold just a single political rally. Little political signage is on display in the city other than the pink banners of Yameen’s Progressive Party of Maldives, sometimes accompanied by towering full-body placards of the leader, elected in 2013.

Aiman Rasheed at Transparency Maldives, a Malé-based election monitoring group, said: “Since we had our first multiparty election in 2008, this one would be the worst in terms of transparency and the level of brazen partisanship.”

The opposition claims the rules for counting Sunday’s votes were abruptly changed this week, including to deny poll observers the chance to scrutinise individual ballots, making it harder to detect vote rigging. Officials from the state-controlled election commission have denied the changes will affect the counting process.

Yameen, 59, has also been accused of trying to cow Raajje TV, the only opposition-aligned television channel, which was fined the equivalent of nearly $130,000 (£100,000) in August for broadcasting a “defamatory” speech from an opposition rally.

Meenakshi Ganguly, the south Asia director for Human Rights Watch, said: “There are serious concerns that President Yameen wants to cling to power by any means, whether by getting rid of the competition in politically motivated criminal cases, by locking up anyone who comes in the way, even judges, or by interfering in the election process itself.”

The opposition coalition has nominated a senior MP, Ibrahim Mohamed Solih, to run against Yameen on a platform of restoring democracy, tackling corruption and improving relations with the west.

Main opposition leader Ibrahim Mohamed Solih greets a crowd at a rally in the Maldives capital Malé. Photograph: -/AFP/Getty Images

Though it is best known for its high-end tourist resorts, several major global faultlines run through the Maldives, giving the country of fewer than 500,000 people an outsized political prominence.

It is a poster child for the fight against climate change, with rising sea levels threatening many of its 1,200 atolls. Its strategic location has made it a key arena in the contest for regional influence between India and China. It has also emerged as an incubator for jihadism: a greater proportion of Maldivians have joined Isis than any country but Tunisia.

These issues have played out alongside an uneasy democratic experiment starting in 2008, when Mohammad Nasheed became the country’s first freely elected president after 30 years of authoritarian rule.

Nasheed, 51, was forced to quit his office in 2012 in what supporters regard as a coup, and was narrowly defeated by Yameen in a presidential poll the following year. He was convicted of terrorism in a dubious trial in 2015 and now lives in exile in Sri Lanka.

Rights groups say Yameen has gradually eroded checks on his power throughout his five-year term, most prominently in February, when he declared a 45-day state of emergency in response to a supreme court decision quashing the convictions of nine opposition leaders. Police arrested two of the judges along with the country’s former ruler and a supreme court administrator, charging all four under terrorism laws.

The supreme court, reduced to three judges after the arrests, later overturned the decision to exonerate the opposition leaders.

Yameen has brushed off concerns he is abusing his power. “If the accusations about authoritarianism are true, when I go to islands, the people will tell me ‘we are tolerating so much abuse’,” he said on the campaign trail this month. “I won’t see smiles on the faces. No one will come to greet me and shake my hand, if there is tyranny.”

The US warned earlier in September it could apply sanctions against anyone seen to be impeding “free and fair” elections in the country. The European Union issued similar threats in July.