Vietnam, a one-party state with little tolerance for dissent, is working hard to control every aspect of the closely-watched meeting to avoid even the most minor embarrassment on the world stage.

At least three dissidents were hauled off by police Wednesday as they tried to catch a glimpse of the leaders' motorcades in the streets of Hanoi.

"We were just taking photos then seven (plainclothes) men jumped into us and brought us here to the station," said Le Dung, livestreaming from a police post in Hanoi.

He was locked in a room with two other activists, he said, adding: "We are now guarded by only one militiaman, who knows nothing."

His live feed suddenly died late Wednesday and he could not be reached on his mobile phone.

Other dissidents said they were closely followed by plainclothes police or kept under de-facto house arrest with officers standing guard outside their homes.

"They are stationed outside my house and follow me whenever I go out," said Nguyen Chi Tuyen, an activist better known by his online nickname Anh Chi.

"For activists like me, the summit just means more trouble."

Vietnam routinely locks up its critics, and has seen a spike in arrests since a hardline administration came to power in 2016.

Amnesty International called on Trump to press Kim on North Korea's dismal rights record at the summit.

The US president's "silence in the face of relentless and grave human rights violations has been deafening", said Francisco Bencosme, advocacy manager for Asia-Pacific at Amnesty's US branch.

Hanoi authorities deported Kim impersonator Howard X earlier this week after he appeared in public with Trump lookalike Russell White.

On Wednesday another Kim doppelganger, Thai Uthen Lueangsaengthong, was put on a plane home after he went on a walkabout in Hanoi, Thai press reported.