Tanzania’s President John Magufuli is being treated in hospital in Kenya and is in a critical condition, opposition leader Tundu Lissu has told the BBC, citing well-placed sources.
He has had coronavirus and a cardiac arrest, Mr Lissu said he been told.
The BBC has not been able to confirm these reports.
Mr Magufuli, who has not been seen in public for 11 days, has faced criticism for his handling of Covid-19 – his government is refusing to buy vaccines.
The East African nation has not published its coronavirus cases since May.
The 61-year-old president has called for prayers and herbal-infused steam therapy to counter the virus.
Earlier this month, at a funeral for a top presidential aide, Mr Magufuli said Tanzania had defeated Covid-19 last year and would win again this year.
The aide died hours after the vice-president of the country’s semi-autonomous islands of Zanzibar, who was being treated for Covid-19.
Mr Lissu said he had been told that President Magufuli was flown to Kenya for treatment at Nairobi Hospital on Monday night.
There has been no official response from the government, which has warned against publishing unverified information about the Tanzanian leader, who was last seen at an official event in Dar es Salaam on 27 February.
Nairobi Hospital also said it could not comment.
Mr Lissu told the BBC that the government’s silence was fuelling rumours, was irresponsible, and the president’s health should not be a private matter.
It would not be a surprise to Tanzanians that Mr Magufuli had contracted coronavirus as he had been reckless in the face of the virus, he said.
“He has never worn a mask, he has been going to mass public gatherings without taking any precautions that people are taking all around the world,” Mr Lissu told the BBC’s Africa correspondent Leila Nathoo from exile in Belgium.
“This is someone who has repeatedly and publicly trashed established medicine, he’s relied on prayers and herbal concoctions of unproven value.”
The 53 year old alleged that Tanzania’s Finance Minister Philip Mpango was also being treated at the same hospital in Kenya’s capital.
Mr Lissu, who came second in presidential elections for the opposition Chadema party in October with 13% of the vote, said he considered his rival’s reputation to be in complete tatters.
“He’s built a reputation as a patriot, that he doesn’t travel outside the country, that he’s a president for the poor – and he’s refused to do anything to ameliorate the situation in Tanzania by telling people we are fine.”
Last week, the Catholic Church in Tanzania urged people to take Covid-19 precautions more seriously, saying 60 nuns and 25 priests had died in the last two months after showing symptoms of coronavirus.
Mr Lissu first went into exile in 2017 after surviving an assassination attempt. He returned to take part 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.