The quiet gated community, in an upscale part of Brazil’s capital, Brasília, doesn’t see much crime. But these days, plainclothes police are standing guard here, tasked with an unusual mission: keeping close watch over former President Jair Bolsonaro.

Inside one of the luxury homes, Mr. Bolsonaro, wearing an ankle monitor, has been under house arrest for weeks. He is awaiting a trial that is to start on Tuesday, in which he is accused of plotting to stage a coup after losing the 2022 presidential elections. He denies the charges.

Now the Brazilian authorities have moved to further tighten the leash on the former leader, amid growing concerns that he may attempt to flee, according to a high-level police official who spoke anonymously about sensitive discussions.

Last week, plainclothes police officers were stationed just outside the gated community, monitoring entry into the complex. Then on Saturday, Alexandre de Moraes, the Supreme Court justice overseeing the case, decided to allow the police to set up around Mr. Bolsonaro’s house, although he refused to let them inside it.

In his ruling, Justice Moraes said the measure was intended to address the risk of escape through “adjacent properties on both sides and at the back, which causes the existence of blind spots.” Ankle monitors like Mr. Bolsonaro’s do not provide the wearer’s movements in real time, a shortcoming that, the police say, could give him time to flee to a neighboring house and out the back of the complex.

The police officials said that there was no intelligence suggesting an imminent escape plan, but that the extra measures were deemed necessary because Mr. Bolsonaro has, on more than one occasion, signaled that he has considered fleeing. The authorities are especially concerned by recent efforts by one of his sons to lobby the White House to intervene in his father’s case, which President Trump did this summer.