As of Tuesday, Tanzania's President John Magufuli has not been seen in public for 17 days and rumours have spread that he is ill.
The speculation has led to several arrests, as the government seeks to contain the rumours.
On Tuesday, the opposition leader from the Alliance for Change and Transparency (ACT Wazalendo) issued a statement demanding Magufuli's whereabouts.
"ACT- Wazalendo calls upon the Government of the United Republic of Tanzania to fully explain the mystery surrounding the absence of President John Pombe Magufuli who has uncharacteristically not been seen in public for over two weeks now," the party leader Kabwe Z. Ruyagwa Zitto said in the statement.
The party also called for the release of all citizens who have been arrested for circulating rumours about the president's health.
There has been speculation and rumours flying on social media that the 61-year-old president may have contracted coronavirus and been airlifted to a Kenyan hospital for treatment then flown to India a day later.
Tanzania's main opposition leader Tundu Lissu, who is exiled in Belgium, said, citing sources that Magufuli was gravely ill from Covid-19, exacerbated by underlying health conditions.
On Tuesday he posted photos on his Twitter account of activity in Dar es Salaam.
'Eradicating coronavirus with prayer'
Magufuli is one of Africa’s most prominent Covid-19 sceptics.
Last year he said the disease had been eradicated from Tanzania by three days of national prayer.
He cast doubt on tests to determine Covid-19 discouraged Tanzanians from wearing masks, keeping a distance from others, and the use of vaccines to combat the disease.
Prime Minister Hassan Majaliwa last Friday denied reports Magufuli is ill and said he is "fit and working hard as usual."
“The president has a lot to do,” he said, adding that the president had delegated some duties to his aides.
On Monday, vice-president Samia Suluhu Hassan dropped a possible hint the president is ill, without naming him.
"Our country is now full of rumours from outside but that should be ignored... It's quite normal for a person to contract flu, fever or any other disease," she said.
"If there's need for us to remain united, the time is now."
Magufuli last appeared in public on February 27, and the fervent Catholic has missed three Sunday services, where he would often address the congregation.
The government has started cracking down on people whom it accuses of spreading the "rumours" that Magufuli was ill through social media, threatening to charge them with cybercrime offences.
So far police have announced the arrest of one person in Dar es Salaam and two in the northern Kilimanjaro region for spreading rumours.
“There are a lot more who have been spreading false statements. We will arrest them all,” regional police commissioner for Kinondoni, Ramadhani Kingai said on Saturday.
Human rights groups say the cybercrime law is one of several that Magufuli’s government uses to suppress freedom of expression.