Three Russian opposition activists were detained Saturday as they travelled to Belarus to observe a tense presidential election there, the group said.

Andrei Pivovarov and two other members of Open Russia, an opposition group established by self-exiled Kremlin critic Mikhail Khodorkovsky, were taken off a bus in the western Pskov region, the group said.

Pivovarov, Open Russia executive director who is a prominent Kremlin critic, and two activists “have been detained by Russian border guards without any explanation,” the group’s chairperson Anastasia Burakova said in a statement on the Telegram messaging app.

Several hours later Pivovarov resurfaced, still detained, in the neighbouring Smolensk region, said Burakova, adding that he was accused of disobeying border guards.

He could face up to 15 days in jail, Burakova added. 

Belarus holds a presidential election on Sunday with a woman opposition candidate posing the greatest challenge in years to long-ruling strongman Alexander Lukashenko.

The 65-year-old leader has headed the ex-Soviet country bordering Russia since 1994 and Sunday’s polls could hand him his sixth term.

Opposition candidate Svetlana Tikhanovskaya has alleged that Lukashenko plans to rig the vote in the absence of international monitors. 

Early voting began Tuesday and turnout already stands at more than 32%, according to official figures.

In the run-up to the polls, Lukashenko accused Russia of meddling, saying Moscow sent 33 mercenaries to foment unrest.

But Kremlin critics say that there is no doubt that the Kremlin supports Lukashenko and Pivovarov’s detention was just the latest example of that.

“A dictator will always support a dictator,” opposition politician Dmitry Gudkov said on Facebook.