Another candidate has been in custody since June after being charged with "breaching internal state security"
"I can confirm 100 percent that Mr (Andre) Okombi Salissa has been arrested," the source said.
Okombi Salissa, 55, had been placed under house arrest three months after the March 2016 vote, which swept Sassou Nguesso back into power.
He then escaped and went into hiding for several months.
In December, prosecutor Andre Ngakala Oko asked Congo's National Assembly to lift Okombi Salissa's parliamentary immunity, to enable the courts to prosecute him for illegal possession of weapons of war.
Along with four other opposition figures, Okombi Salissa had sought to stop 73-year-old Sassou Nguesso from returning to power in March.
Another candidate, General Jean-Marie Michel Mokoko, has been in custody since June after being charged with "breaching internal state security" and illegal possession of weapons.
Although he was minister for several portfolios under Sassou Nguesso from 1997 to 2012, Okombi Salissa joined the opposition shortly before a constitutional referendum in 2015.
Congo has been on edge since the referendum ended a two-term limit on presidential mandates, allowing the head of state to extend his three-decade grip on power.