State channel STV cited the powerful KGB security agency as saying that the arrests were part of a probe into the preparation of mass unrest.
Earlier Vyasna, an NGO that defends the rights of political prisoners, said 17 people were detained in this week's wave of arrests in the ex-Soviet country bordering Russia and the European Union.
The EU called for their immediate release and said it was in contact with Minsk authorities, urging restraint.
In recent weeks, thousands have protested against Lukashenko's rule and a controversial new tax on "spongers" - those who work less than six months a year - as the country suffers an economic slump.
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The biggest round of opposition protests yet is planned for Saturday.
Television reports claimed the detainees were planning to stage "provocations" during Saturday's rally and that a weapon cache had been uncovered.
The latest arrests began overnight Tuesday to yesterday after Lukashenko said police had detained several dozen people who underwent weapons training at camps in Belarus and elsewhere.
The detainees are being held in KGB-controlled prison and have not been allowed to meet with lawyers, Vyasna said.
It said some are being held on suspicion of training people to participate in mass protests involving violence or armed resistance.
Yesterday, Vyasna said at least six people had been detained but upped the number on Thursday to 17.
Belarusian state-controlled television yesterday showed an activist detained with a replica hand grenade and a firearm as well as the Ukrainian Insurgent Army (UPA) flag.
The UPA was a group of Ukrainian nationalists who fought against Soviet troops in World War II alongside Nazi forces.
Another of the detainees was identified by Vyasna as bookseller Miroslav Lozovsky, who had previously been linked to White Legion.
Dozens of protesters, including journalists and opposition leaders, were hauled in after the recent protests, receiving fines or up to 15 days in police cells.
The European Union last year lifted most of its economic sanctions against Belarus after Lukashenko, who has led the country since 1994, took steps to appease the West, including by releasing political prisoners.