(AFP) - Investigators in the Democratic Republic of Congo have uncovered "serious indications of actions against state security", the government said Tuesday following the surprise weekend arrest of the president's special security adviser.

"The investigations are ongoing and they are taking place at various levels," a spokesman of President Felix Tshisekedi said in a statement read over state television, urging the Congolese people to "stay calm".

Francoise Beya, who had been considered a powerful figure in the conflict-riven Central African country, was arrested on Saturday while Tshisekedi was attending an African Union summit in the Ethiopian capital Addis Ababa.

The arrest sparked protests, fears of instability within the Congolese government and even rumours of an attempted coup.

A security source told AFP on condition of anonymity that Beya was "suspected of wanting to try something in the absence of the head of state" on the weekend.

In Tuesday's statement, presidential spokesman Tharcisse Kasongo Mwema said: "No attempt at destabilisation of our democratic institutions will be tolerated."

The authorities have provided no information on Beya's arrest.

Mwema said that the National Intelligence Agency (ANR), where Beya is being questioned, does not generally "communicate on its activities".

"However... we can confirm that the investigators have indications of serious actions against national security."

"Investigations are being carried out at different levels," he said, adding that the government would communicate further "if it is necessary".

- 'Situation is under control' -

Mwema also called on the population to "be vigilant and avoid giving credence to false speculation disseminated by ill-intentioned people in the media and on social networks".

"The indications are sufficiently serious and do not point to other considerations of a tribal, clan or regional nature," he said.

DR Congo has a long history of political and ethnic violence.

The east has been plagued by fighting between armed groups for a quarter of a century.

Beya, 67, served as the head of migration under long-ruling former president Joseph Kabila until 2019, when Tshisekedi won election in the first peaceful handover of power since the country won independence from Belgium in 1960.

Beya remained in the key security post when Kabila -- who had ruled since taking over from his father in 2001 -- and Tshisekedi fell out badly in late 2020.

Tuesday's presidential statement stressed that the "democratic process initiated by the first peaceful transfer of power in January 2019 is a sacred achievement to be preserved at all costs".

"The situation is under control," the statement added.

Tshisekedi had travelled to Addis Ababa to attend a two-day African Union summit, which ended on Sunday with leaders condemning a recent "wave" of military coups on the continent.

The president returned home on Saturday night, with his entourage denying that he had cut short his stay.

- Pro-Kabila MP arrested -

The arrest of the key security adviser comes as the Congolese government struggles to stop the bloodshed from conflict with armed groups raging in the country's east.

In May last year, Tshisekedi placed the North Kivu and Ituri provinces under a "state of siege".

On Tuesday, a local deputy from the Common Front for Congo (FCC) party that supports Kabila was arrested in North Kivu accused of inciting disobedience against the state of siege.

Josue Mufula said he was removed from his flight from provincial capital Goma to Kinshasa.

"I was taken off my flight, I don't know why... military terror reigns," he said in a message sent to journalists.

At a military court, prosecutors accused him of passing out leaflets on the weekend reading "bye bye state of siege" as well as insulting police officers.

Another provincial deputy was arrested in Goma on Sunday in similar circumstances.

© Agence France-Presse