Pressure is mounting on Sri Lanka’s president to allow parliament to decide between two men claiming to be the country’s lawful prime minister, amid warnings that the constitutional dispute could lead to a “bloodbath”.

President Maithripala Sirisena announced on live TV on Friday that Ranil Wickremesinghe, with whom he had governed in a fragile coalition since 2015, had been dismissed. In his place he has appointed the former president Mahinda Rajapaksa.

On Monday a cabinet minister was arrested and there were long queues outside petrol stations in the capital, Colombo, as the crisis entered its third day.

Wickremesinghe continued to occupy his official residence, and hundreds of supporters gathered outside, while Rajapaksa took control of the prime ministerial office and starting naming a cabinet.

“At the moment there is a vacuum, no one is in full charge of the country,” Wickremesinghe said during a press conference at the residence.

Wickremesinghe argues he cannot legally be dismissed until he loses the support of parliament. His party, which holds a plurality of seats in the 225-member assembly, was prevented from holding a vote when Sirisena abruptly suspended parliament on Saturday until 16 November.

Wickremesinghe said in a Facebook post on Monday he had obtained the signatures of 126 MPs calling for parliament to be returned immediately to end the political standoff.

“We want parliament summoned immediately to decide who enjoys the majority,” Wickremesinghe said. “I am still the prime minister who commands that majority.”

The prime minister’s office in Colombo, Sri Lanka. Photograph: MA Pushpa Kumara/EPA

The US state department called on Sirisena to immediately reconvene parliament to allow MPs to pick a prime minister.

The parliamentary speaker, Karu Jayasuriya, a member of Wickremesinghe’s party but officially neutral, said: “We need to solve this through parliament. If we try to solve this in the streets it will lead to a major bloodbath.”

Wickremesinghe said he expected Jayasuriya to recall parliament within two days, but it is unclear whether the power to do so lies with the speaker or the president.

Rajapaksa issued his first statement since his appointment on Sunday evening, saying a change of government was required to restore stability. “A wave of crime has hit the country,” he said. “The economy of the country is deteriorating by the day.”

Arjuna Ranatunga, the petroleum minister, was detained by Colombo police a day after one of his guards fired into a mob of unionists allied to Sirisena, killing one man and injuring three. He was released on bail on Monday night.

Ranatunga had earlier told a press conference that union members had mobbed him as he tried to enter a government building and he had feared for his life. His arrest was demanded by petroleum unions who called a strike after the shooting, prompting panic-buying in Colombo.

Supporters of Wickremesinghe as well as Rajapaksa have paid high-profile visits to influential Buddhist monks in recent days trying to court popular legitimacy for their rival sides.

Behind the scenes, both camps are also working to secure their numbers in parliament, with loyalties shifting back and forth unpredictably. One MP, Vasantha Senanayake, pledged his support for Rajapaksa on Saturday, then switched to Wickremesinghe on Sunday – and was then named in Rajapaksa’s cabinet on Monday night.

Wickremesinghe said in a speech broadcast on Facebook that his government had “worked hard to regain the freedoms that were lost until 2015”.

“We established the Right to Information Act, independent commissions and an unprecedented democratic culture. Let us not allow these hard-won freedoms to be taken away from us again,” he said.

Rajapaksa’s decade in power was marred by allegations of authoritarianism, corruption and human rights abuses, especially against the country’s Tamil minority.

He was defeated at the 2015 presidential elections when Wickremesinghe and Sirisena formed an unlikely coalition, and their government initiated several investigations into alleged Rajapaksa-era crimes.

The inquiries’ progress will almost certainly stall if Rajapaksa returns to power, while Tamil groups fear that an uneasy reconciliation process that began after the end of the 2009 civil war could also be imperilled.