HANOI, Vietnam (AP) — President Barack Obama on Tuesday pressed Vietnam to allow greater freedoms for its citizens, arguing that better human rights would improve the communist country's economy, stability and regional power.

On his second full day in the southeast Asian nation, Obama met with activists, including pastors and advocates for the disabled and sexual minorities, to underscore U.S. support for improved rights. Yet a handful of others were prevented from meeting with Obama, prompting the White House to protest to Vietnam's government.

One of those denied access to the meeting was Nguyen Quang A, an economist who had tried unsuccessfully to be selected to the National Assembly. He said that when he left his home to attend the meeting, security men grabbed his hands and legs, threw him in a car and drove him into the countryside, then held him there until Obama had left town.

"They told my son, 'Your father, we have to block him," A said in an interview.

Obama took note of those denied access to the meeting, but said that while "there are still areas of significant concern," the country has made "remarkable strides in many ways."

His visit to Vietnam included the lifting of one of the last vestiges of Vietnam War-era antagonism: a five-decades-old arms sale embargo. In a speech at the National Convention Center, Obama sought to balance a desire for a stronger relationship with Vietnam with efforts to hold its leadership to account over what activists call an abysmal treatment of government critics.

Nations are more successful when people can freely express themselves, assemble without harassment and access the internet and social media, Obama said.

"Upholding these rights is not a threat to stability but actually reinforces stability and is the foundation of progress," Obama told the audience of more than 2,000, including government officials and students from five universities across the Hanoi area. "Vietnam will do it differently than the United States does ... But there are these basic principles that I think we all have to try to work on and improve."

Freedom of expression is where new ideas happen, Obama said. "That's how a Facebook starts. That's how some of our greatest companies began."

Obama's deputy national security adviser, Ben Rhodes, told reporters that a number of activists set to meet with Obama were either prevented from doing so or made to feel uncomfortable attending, "using a variety of different methods." He said U.S. Secretary of State John Kerry and White House officials raised the issue with Vietnam, adding that the U.S. would follow up to ensure those activists are free and aren't being punished.

"Clearly this was something that was the source of significant discomfort for the government," Rhodes said of Obama's meeting with activists.

Obama also said that journalists and bloggers can "shine a light on injustice or abuse" when they are allowed to operate free of government interference or intimidation. He said stability is encouraged when voters get to choose their leaders in free and fair elections "because citizens know that their voices count and that peaceful change is possible."

The president also traced the transformation of the U.S.-Vietnamese relationship, from wartime enemies to cooperation. He said the governments are working more closely together than ever before on a range of issues.

"Now we can say something that was once unimaginable: Today, Vietnam and the Unites States are partners," he said, adding that their experience was teaching the world that "hearts can change."

He referred in the speech to China's growing aggression in the region, something that worries many in Vietnam, which has territorial disputes in the South China Sea with Beijing.

Obama got a round of applause when he declared that "big nations should not bully smaller ones," an allusion to China's attempt to push its rivals out of disputed territory. Obama said the United States will continue to freely navigate the region and support the right of other countries to do the same.

After Hanoi, Obama flew to Ho Chi Minh City, formerly Saigon. He visited the Jade Emperor Pagoda, considered one of the most beautiful pagodas in southern Vietnam and a repository of religious documents that includes more than 300 statues and other relics. A strong smell of incense hung in the air as visitors frequently burn incense outside the main temple to announce to the heavens their arrival.

As Obama paused before one statue, a guide explained that if he wanted to have a son, he should pray to her.

"I like daughters," Obama replied.