” But several activists who had been scheduled to meet with him before the speech were prevented from doing so, underscoring the gulf with Hanoi on human rights.

The White House had requested the meeting as a signal to Vietnam’s Communist government that the United States cares about human rights here. Mr. Obama spent more than his allotted time with the six Vietnamese civil society leaders who did attend the meeting at a JW Marriott hotel, but he said that several others had been prevented from coming.

“Vietnam has made remarkable strides, the economy is growing quickly, the internet is booming, and there’s a growing confidence here,” Mr. Obama said when a group of reporters were briefly allowed into the meeting. “But as I indicated yesterday, there’s still areas of significant concerns in terms of areas of free speech, freedom of assembly, accountability with respect to government.”

Human rights activists, who criticized Mr. Obama on Monday for lifting a decades-old arms embargo against Vietnam without obtaining concessions on human rights, said that Vietnam’s actions on Tuesday proved their point.

“Vietnam has demonstrated itself that it doesn’t deserve the closer ties the U.S. is offering,” said John Sifton of Human Rights Watch. “Detaining or preventing civil society from meeting President Obama is not just an insult to the president, it’s also a human rights abuse in itself, a deprivation of the right to freedom of expression and freedom of movement.”

The activists kept from the meeting included Nguyen Quang A, 69, a businessman who had tried to run this year as an independent candidate for Parliament but was disqualified by the government.

He had been detained by plainclothes security officers, he said later by telephone. They shoved him into a car outside his home in Hanoi at 6:30 a.m. Tuesday, confiscated his cellphone, preventing him from contacting his family, and then drove him 50 miles east of Hanoi.

“I was taken on a touristic tour,” he said. The men declined to say why they were driving him around for seven hours, just saying to him, “You know why we have to do this.”

A prominent blogger and journalist, Pham Doan Trang, who had flown to the Vietnamese capital from Ho Chi Minh City on Monday, was also barred from attending. She had not been heard from since landing in Hanoi, according to Phil Robertson, deputy director of the Asia division of Human Rights Watch.

Ha Huy Son, a lawyer who specializes in defending dissidents in court, was also kept from the meeting. “Security people have been guarding me at my home for the last two days,” he told Agence France-Presse, saying he had been told he could go anywhere but to the embassy.

The use of security forces to keep the activists from an event that the Vietnamese government had agreed to suggests that the government may have been divided over the meeting. It is unusual for a government, even one with a poor record on human rights, to allow such a gathering with an American president to proceed and then prevent some guests from attending.

Benjamin Rhodes, a deputy national security adviser, said that administration officials became aware Monday night that the government was preventing some activists from attending and that American officials had objected.

Still, he defended the decision to lift the arms embargo even if the government failed to immediately improve civil rights.

“We believe that broadly speaking what we’ve done through normalization in Vietnam is empowering the Vietnamese people,” he said, adding: “And that we can push that process forward much more effectively by deepening the relationship than by pulling back.”

Mr. Obama also took advantage of his afternoon speech at the National Convention Center here to make the case for improving human rights in Vietnam.

The audience of about 2,300 well-dressed Vietnamese, seated in red velvet seats and almost certainly vetted by the government, cheered loudly when Mr. Obama appeared. They cheered again when he said, “Vietnam is an independent and sovereign nation, and no other nation can impose its will on you,” an apparent reference to China, which has claimed much of the seas just off Vietnam’s 2,000-mile coastline.

But the auditorium was notably silent when he broached the subject of human rights practices.

He said that the United States was not trying to impose its form of government on Vietnam, but that some values were universal. The rights to free speech, assembly and a free press, he said, were enshrined in Vietnam’s Constitution.

“So, really, it’s about all of us, each country, trying to consistently apply these principles,” Mr. Obama said. “Making sure those of us in government are being true to those ideals.”

Human Rights Watch estimates that about 110 political dissidents are serving prison sentences in Vietnam. In March, Nguyen Huu Vinh, 60, a blogger, was sentenced to five years in jail for writing posts that were deemed to be against the government.

By Gardiner Harris & Jane Perlez - The New York Times - May 24, 2016