MINSK, Belarus  Ever since riot police officers crushed a large protest against apparent fraud in presidential elections here last month, the security services  still called the K.G.B. in this authoritarian former Soviet republic  have been rounding up people across the country for even the most tangential affiliation with the opposition.

Now, it seems, they have gone a step further.

The government warned recently that it might seize custody of the 3-year-old son of an opposition presidential candidate who was jailed along with his wife, a journalist. The authorities said that they were investigating the status of the child, who is now living with his grandmother, and that they expected to make a decision by the end of the month.

In 16 years as ruler of Belarus, President Aleksandr G. Lukashenko has often been called Europe’s last dictator. But the plight of the child, Danil Sannikov, may represent a new tactic in the government’s persecution of the opposition, one that harks back to the Stalin era, when the children of so-called enemies of the people were sent to orphanages after their parents went to the gulag.

“Even in my worst nightmares I could not have conceived that this could happen,” said the child’s grandmother, Lyutsina Khalip.