HANOI, Vietnam — While President Barack Obama pressed his Vietnamese hosts Tuesday to ease their human rights abuses, as if to illustrate his point, some dissidents were barred from meeting with him in Hanoi.

Speaking to university students, Obama urged Vietnam's government and people to insist on freedom of speech, assembly and religion, not least as a way to allow new ideas to gain hold and innovations to help the economy thrive.

“Upholding these rights is the fullest expression of the independence that so many cherish, including here, in a nation that proclaims itself to be of the people, by the people and for the people,” Obama said, citing a line from the Vietnamese Constitution that echoes Abraham Lincoln's Gettysburg Address.

But human rights advocates were outraged that three dissidents were barred or discouraged from meeting with Obama, pointing to their absence as a sign that Vietnam hasn't earned the warming of relations and strengthened business relationships that Obama is in this country to promote.

“It's like the government is putting on a demonstration for Obama of their repressive governance,” said John Sifton of the group Human Rights Watch.

The day provided a vivid illustration of the conflict at the heart of Obama's mission to the U.S.'s former Cold War enemy this week, as well as to other authoritarian nations he sees as on the brink of possible reform.

Though nations including Vietnam, Myanmar and Laos are expanding their economies rapidly and presenting themselves as potential U.S. partners in trade and global security, Obama's decision to engage with their governments links him with leaders at odds with dearly held American values like freedom of speech, religion and assembly.

On Tuesday, government agents reportedly pressured the three activists to keep them from sitting down with Obama, according to U.S. officials and Human Rights Watch.

Journalist Doan Tran was arrested on Monday, Sifton said. Nguyen Quang, an economist and candidate for National Assembly, was detained and prevented from leaving his house. And Ha Huy Son, a lawyer who has represented dissidents, was also prevented from traveling to meet Obama.

White House officials also confirmed that the civil society leaders were stopped from attending the meeting.

Sifton said the episode was more evidence that Obama never should have taken the dramatic step Monday of dropping the Vietnam War-era arms embargo against the communist government without exacting concessions on human rights violations.

“The Vietnam regime has demonstrated today that it doesn't deserve the closer ties the U.S. is offering,” he said.

But Secretary of State John Kerry, a Vietnam War veteran traveling with Obama this week, said the incident showed why U.S. engagement with the country was so critical. He said the meeting with dissidents, albeit without the full participation the U.S. had sought, “could not have been imagined 20 years ago,” when the U.S. first took steps to normalize relations with Vietnam. And it only could have happened because of Obama's direct advocacy with his counterparts.

“That's at the center of President Obama's foreign policy,” Kerry said, noting the approach here was also being applied to Cuba and Myanmar as well as Iraq and Afghanistan.

The kind of transformation Americans might expect will not take place overnight, he added.

“We have to recognize the road that they're on is going to be a roller coaster ride to some degree,” he continued. “But as long as it's moving in the same direction, that's what is important, as long as the United States itself remains faithful to our values.”

It was significant, White House aides noted, that Obama himself publicly acknowledged the absence of some dissidents from the meeting. He said it showed that while there has been some “modest progress” on human rights, many still lack the full freedom to express dissent.

The deepening of ties seen on Obama's visit, including his lifting of the ban on the sale of lethal weapons, was an important step, said Sen. Lindsey Graham, R-S.C., even if he would have liked to see more done on human rights.

“Hopefully, this, over time, gives us more leverage to influence how Vietnam treats its people,” he said.

Obama must continue to press the Vietnamese on these issues, said Sen. John McCain, R-Ariz., a former Vietnam prisoner of war who, along with Kerry, helped pave the way for normalization of relations.

The Vietnamese government has not made enough progress on human rights, said McCain, who has repeatedly traveled to Vietnam and has regular contact with its leaders.

“Every time I meet with them, I mention that,” he said. “And strangely enough, they seem to acknowledge it.”

Michael A. Memoli in Washington contributed.

cparsons@tribune.com