BLANTYRE, Malawi: Malawi police on Monday arrested 11 people, including the former president's brother and a serving minister, for an alleged coup plot, sparking violent protests.

The group are accused of trying to prevent then vice president Joyce Banda from assuming power last year, after the death of president Bingu wa Mutharika.

Among those arrested was former foreign affairs minister Peter Mutharika -- the brother of the late leader and his heir apparent.

He was charged with perjury and soliciting to break the law at the Lumbazi police station in Lilongwe, his assistant Ben Phiri told AFP.

It was not clear if the group would also face the more serious charge of treason, which could carry the death sentence.

Police and government sources said the others detained included the former ministers for local government, information, finance and sports as well as top civil servants.

The current minister of economic planning, Goodall Gondwe, was also among those arrested.

"It's not political. It's not about Joyce Banda," Information Minister Moses Kunkuyu said, adding that the arrests were the natural consequence of an ongoing investigation.

Also among those arrested was chief secretary to the government Bright Msaka.

The presidency swiftly announced he had been sacked and replaced by his deputy Hana Ndilowe.

The arrest of the "coup plotters" sent shock waves through Malawi, fuelling anger at Banda's government, which has been much feted by the international community.

Malawi police fired teargas at around 500 people protesting the arrests outside police headquarters in Blantyre, but the protesters failed to disperse and retaliated by throwing stones.

Another group of protesters, estimated to be in the thousands, gathered nearby, blocking the main highway to the capital Lilongwe with boulders and branches.

Banada called an emergency cabinet meeting to discuss the crisis.

The plot is alleged to have taken place amid the chaos of Bingu wa Mutharika's death in April last year.

Last week an inquest ruled that Mutharika died of a heart attack on the way to hospital after collapsing at State House.

But his death was not confirmed for two days and his body was flown to South Africa as would-be successors sought to buy time.

Six ministers, including then local government minister Henry Mussa, held a late-night news conference a day after Mutharika's death to insist he was still alive.

At the time Banda was the vice president, who under the constitution was next in line to become the president.

But Mutharika had been grooming his brother Peter to succeed him.

Banda was eventually sworn in on April 7, after backroom dealings.

A month after coming to power Banda alleged the coup plot was "against this country and not me. It's a national issue."

Three of the alleged coup plotters were said to be suffering from hypertension in custody at a Lilongwe police station, according to Hetherwick Ntaba a former spokesperson for Mutharika.

The three were already suffering from "various medical conditions, and you can imagine under the circumstances, their conditions have aggravated," said Ntaba.

Political analyst Ernest Thindwa of the University of Malawi said the publication of the inquest had sparked the arrests.

"The inquiry report raised a number of questions and established that offences were committed. But it's important that the suspects be given a fair trial."

Law expert Fidelis Edge Kanyongolo said the path forward could be rocky for Banda.

"Government must proceed with caution and make sure it has sufficient evidence to secure a conviction, otherwise it will look like political persecution."

Since coming to power, Banda has won plaudits from the international community, and attracted opposition at home, for embarking on a series of painful IMF-inspired economic reforms.

Budget cuts and a currency devaluation have meant living costs for ordinary Malawians have gone up, just as wages in the massive public sector are frozen.

Thousands have taken to the streets to oppose the measures over the past year, which Banda insists are needed to put the economy -- one of the poorest in the world -- back on track.

Agriculture still accounts for 30 percent of GDP in the landlocked country.