Sudan's information minister had one clear message after security agents moved in to arrest their former spy chief - that a plot had been uncovered, the culprits caught and the situation in the country was now "totally stable".

Khartoum did appear quiet a day later on Friday - but on the desert city's dusty streets the detention amplified a debate about the future of the country's leader, and posed new questions about who might one day unseat him.

President Omar Hassan Al-Bashir has kept a near quarter-century hold on this African oil producer, drawing support from its all-powerful military, security services and large parts of the devout, overwhelmingly Muslim population.

His speeches regularly referred to threats from outside - from the West, from Israel and from the International Criminal Court, which has indicted him over charges of atrocities in Darfur.

But the man arrested on Thursday on charges of plotting to "incite chaos" was Salah Gosh, once one of the most powerful figures in Bashir's inner circle, together with several other senior figures from army and security.

"He [Bashir] would always be worried ... that the opposition groups would be involved, but now it's coming from inside. There is a categorical difference there," said Magdi El-Gizouli, a fellow at the Rift Valley Institute.

Gizouli said there were signs of frustration inside the military, where many who had fought Bashir's wars in Darfur and the south were itching for the old guard to move on. "Inside the regime and army are many who want jobs. They are not young anymore. It's not about reforms. It's about power," he added.

Public unrest has also been growing over rising prices and growing inflation after South Sudan split away as an independent nation last year, under the terms of a peace deal, taking most of the country's oil reserves with it.

Diplomats and analysts in Sudan told Reuters they doubted those tensions had built up into an active military coup attempt.

The detentions were rather a warning shot to those inside the army, security and services and ruling party who might even be considering searching for a replacement for Bashir, they added.

"This is a power struggle inside the regime. It was a preemptive measure," said Faisal Mohammad Saleh, a prominent journalist. "I don't think the arrested people had planned anything yet. Maybe they made some phone calls or met, but this was an early stage."

Quieter questions were already circulating about Bashir's future after an official from his ruling National Congress Party said last year, in the wake of Arab Spring protests, he would not seek re-election in the 2015 presidential election.

Speculation mounted after officials were forced to announce Bashir underwent throat surgery in Qatar in August when people in the street started wondering why he had stopped holding his famous public rallies.

Officials insist Bashir is healthy after undergoing a second operation in Saudi Arabia. But a reduction in his public appearances has kept people talking.

At an Islamist conference last week Sudan invited Muslim leaders from Egypt, Tunisia and further afield, but Bashir's seat was vacant for more than one hour at the opening ceremony.  When Bashir finally came, officials interrupted a speech to declare "the president of the republic is here."

The startling arrest of Gosh and the other officials, opened up another dimension in the speculation. "You cannot stop the succession debate anymore. It's there and people wonder how fit their president is or whether he wants to run again at the next elections," said a Western diplomat.

"[Vice President Ali Osman] Taha has many supporters in the NCP but there are others in the army and NCP with ambitions, too."

While details of the plot arrests remain unclear, analysts say the trigger could have been worries that once-loyal Islamists would stage protests after hopes for reforms were dashed at the Khartoum conference last week.

In papers circulated in mosques before the meeting, many had demanded to discuss fighting corruption or reforms at the forum of the Islamic Movement, a quasi-official organisation meant to guide the NCP.

But hopes for changes were dashed when the government managed to push through its candidate, former minister Al-Zubeir Al-Hassan, as new secretary general.

"He is not a historical Islamist leader, he doesn't really have Islamic credentials," said Harry Verhoeven, a long-time Sudan watcher.

Thousands of mid-level army officers heeded calls by Muslim leaders to fight southern "infidels" during the country's decades-long civil war with South Sudan. Many of them were furious when the peace deal that Bashir secured to end that conflict opened the door for southerners to secede from Sudan last year.

Former "mujahideen" have organised themselves into groups such as the "Saihun," or travelers in Arabic, to vent their anger against the government. "I went fighting as a young man and I feel this government has run the country aground with their corruption. We have ministers who are not competent," said an Islamist, speaking to Reuters before the arrests were announced.

After the arrests, Islamists uploaded pictures of one of the army officers detained alongside Gosh - Wad Ibrahim, a civil war hero respectfully called "Emir of the Mujahideen".

Shops opened as usual in the morning before Friday prayers. At one mosque visited by Reuters the preacher did not mention the arrests.

Many Sudanese, used to political upheavals, wondered what will happen next. "Who is ruling this country," wrote a user on the "Saihun" website on Thursday as rumours were swirling around. "I don't think they tried to stage a military coup," said 55-year-old Ibrahim Idriss, a food merchant in central Khartoum. "I'm not interested in it anyway. I worry more about corruption and officials who steal people's money."