Questions were raised about the health of Tanzanian President John Magufuli, who has not been seen in public for 11 days.
Opposition leader Tundu Lissu told the BBC that, according to his sources, the president is being treated at the hospital for coronavirus in Kenya.
The BBC was unable to independently verify this report.
Magufuli has faced criticism for dealing with Covid-19, with its government refusing to buy vaccines.
The East African nation has not published its coronavirus cases since May.
Its 61-year-old president called for prayers and herbal steamed steam therapy to fight the virus.
Earlier this month, at the funeral of a top presidential aide, Magufuli said that Tanzania had defeated Covid-19 last year and would win again this year.
The aide died hours after the vice president of the semi-autonomous islands of the country of Zanzibar, who was receiving treatment for Covid-19.
‘Irresponsible silence’
Lissu said he was told that President Magufuli had flown to Kenya for treatment at Nairobi Hospital on Monday night.
According to the opposition leader, the president suffered a cardiac arrest and is in critical condition.
There was no official response from the government, which warned against publishing unverified information about the Tanzanian leader, who was last seen at an official event in Dar es Salaam on February 27.
Nairobi Hospital also said it could not comment.
Another Tanzanian politician told the Associated Press, on condition of anonymity because he feared an adverse reaction, that he had spoken to people close to the president who said he was seriously ill and hospitalized.
Lissu told the BBC that the government’s silence fueled rumors and was irresponsible, and that the president’s health should not be a private matter.
It would not come as a surprise to Tanzanians that Magufuli had contracted the coronavirus because he was reckless in the face of the virus, he said.
“He never wore a mask, he has gone to large public meetings without taking the precautions that people are taking around the world,” Lissu told the BBC, from exile in Belgium.
“This is someone who has repeatedly and publicly destroyed established medicine, he has relied on prayers and herbal mixtures of unproven value.”
The 53-year-old man claimed that Tanzanian Finance Minister Philip Mpango was also being treated at the same hospital in the Kenyan capital.
Analyze
By Leila Nathoo, BBC Africa correspondent
It would be an explosive development if it were confirmed that President Magufuli suffers from Covid-19 – the disease from which he spent months denying the threat. It is worth reiterating that there has still been no official response or comment on his status or whereabouts.
In addition to the potential implications for the Tanzanian government if it is in fact seriously ill, for its critics, diagnosis and treatment abroad would be proof of a failed disease strategy.
In the absence of any official data, there is no way to know the true extent of coronavirus infections in Tanzania, but in recent weeks there has been growing concern about an increase in cases.
If the president were among those recently infected, it would be a grim revenge for those who have been warning not to ignore the spread of the virus.
Lissu, who came in second in the presidential election for the opposition Chadem party in October, with 13% of the vote, said he considered his rival’s reputation in tatters.
“He has built a reputation as a patriot, who does not travel abroad, who is a president of the poor – and has refused to do anything to improve the situation in Tanzania, telling people that we are okay.”
Last week, the Catholic Church in Tanzania urged people to take Covid-19 precautions more seriously, saying that 60 nuns and 25 priests had died in the past two months after showing symptoms of coronavirus.
Mr. Lissu went into exile for the first time in 2017, after surviving an assassination attempt. He returned to participate in last year’s polls, the results of which he says were rigged.
He left the country again in November, saying he had received more death threats.