MINSK — A Belarussian human rights organization said Wednesday that seven of the candidates who ran against President Alexander Lukashenko could face up to 15 years in prison after post-election violence and massive arrests.

Belarus' security service, which is still called the KGB, has filed charges against 20 top opposition figures, including the seven presidential candidates, for organizing mass disturbances, said Ales Belyatsky of the human rights center Vesna.

KGB spokesman Alexander Antonovich declined comment.

Some 700 people, including the candidates, were arrested after Sunday's election that gave Lukashenko a fourth term in office.

Two of the arrested candidates were later released, but both of them — Grigory Kostusyev and Dmitry Uss — were summoned to KGB offices for further questioning on Wednesday.

Many of the arrests came on Sunday night, when thousands protested the results. The international monitors called the election fraudulent.

In a brief telephone interview, Kostusyev said that "the regime has shown its true essence."

"We've been thrown 10 years into the past," he said.

Kostusyev said Uss was sitting next to him in the KGB offices as he spoke.

Along with Uss and Kostusyev, those charged include Andrei Sannikov, whom the Central Elections Commission's widely disputed count says was the top vote-getter among the challengers — credited with 2.4 percent compared with Lukashenko's 79.6 percent. Sannikov's wife, Irina Khalip, and Natalya Radina, editor of an opposition web site affiliated with Sannikov, also face the charges, Vesna said.

Vladimir Neklyayev, another prominent challenger, was beaten as he tried to lead a column of supporters to the protest in central Minsk on Sunday night. He was hospitalized, but later forcefully taken from the hospital and placed in KGB custody.

The other arrested candidates are Nikolai Statkevich, Vitaly Rymashevsky and Ales Mikhalevich.

Also Wednesday, the Belarussian parliament ratified an agreement to create a "unified economic space" with Russia and Kazakhstan.

Preliminary agreement on the zone was reached 10 days before the election. As part of that agreement, Russia said it would drop tariffs on oil exported to Belarus, a concession that significantly strengthened Lukashenko's hand.