The detention of Karim Massimov was announced by the National Security Committee which he headed until he was fired by President Kassym-Jomart Tokayev on Wednesday after violent protests swept across the Central Asian country.
Tokayev's office said he had told Russian President Vladimir Putin in a phone call that the situation was stabilising.
"At the same time, hotbeds of terrorist attacks persist. Therefore, the fight against terrorism will continue with full determination," it quoted him as saying.
The Kremlin said Putin backed Tokayev's idea to convene a video call of leaders from the Collective Security Treaty Organisation (CSTO), under whose umbrella Russia and four other former Soviet republics have sent troops into Kazakhstan to help restore order.
It was not clear when this would take place.
Dozens of people have been killed, thousands have been detained and public buildings across Kazakhstan have been torched over the past week in the worst violence experienced in the oil and uranium producer since it became independent in the early 1990s as the Soviet Union collapsed.
Tokayev has ordered his troops to shoot to kill to end what he has called attacks by bandits and terrorists.
He said on Friday the state had "slept through" instigators' preparations to launch attacks on the biggest city, Almaty, and across the country.
Massimov's arrest indicated moves were underway against those deemed responsible.
Apart from heading the intelligence agency that replaced the Soviet-era KGB, Massimov was twice prime minister and worked closely with former president Nursultan Nazarbayev, the country's ruler for three decades until he turned over the presidency to Tokayev in 2019.
There were no details of the treason allegations.
The security service said other officials were also detained but did not name them.
On Friday, a pro-government politician said on television he had information that the security forces had been ordered to abandon Almaty airport so protesters could take it over.
He said they had left a security building in the city undefended, enabling people to seize weapons.
It was not immediately possible to verify this account.
The airport remains closed but is now under the control of Kazakh security personnel and Russian troops, according to Russia's defence ministry.
The demonstrations began as a response to a fuel price hike but swelled into a broad movement against Tokayev's government and 81-year-old Nazarbayev.
Tokayev removed Nazarbayev on Wednesday as head of the country's Security Council, a role in which he had continued to wield significant influence.
Interfax news agency reported on Saturday that the council's deputy head Asamat Abdymomunov had also been fired.
In Almaty, where security forces have reclaimed control of the streets since Friday, a Reuters reporter said occasional gunshots were heard on Saturday.
Some businesses began to reopen in the city as people ventured out to buy supplies, and queues formed at petrol stations.
Security forces patrolled the streets and set up checkpoints.
The deputy mayor was quoted by Russia's RIA news agency as saying operations to purge the city of "terrorists and bandit groups" were still underway and citizens were advised to stay at home.
Zhumadin Patov, the deputy head of a public market in Almaty, said the checkpoints and petrol station closures had complicated food distribution in the city of about two million people.
"There is enough food in warehouses but it cannot be delivered because of the checkpoints and lack of fuel," he said.
In the capital Nur-Sultan, Reuters filmed police stopping drivers at a checkpoint with armed soldiers nearby.
The interior ministry said more than 4400 people had been detained since the start of the unrest.
Tokayev announced a day of mourning for Monday to commemorate those killed.
Access to the internet, which was been largely shut down in Kazakhstan for days, was still heavily disrupted on Saturday.
The deployment of the Russia-led CSTO military alliance at Tokayev's invitation comes at a time of high tensions with NATO as Russia and the United States prepare for talks next week on the Ukraine crisis.
