Secretary of State Marco Rubio estimated that he had signed perhaps more than 300 letters revoking the visas of students, visitors and others to force their expulsion from the United States because of their foreign policy views or criminal activities.

He has been signing letters daily to revoke visas since taking office in late January, Mr. Rubio told reporters on Thursday night aboard an Air Force passenger jet traveling between Paramaribo, Suriname, and Miami, where he lives with his family. Mr. Rubio was concluding a three-nation tour in the Caribbean and South America.

“I don’t know actually if it’s primarily student visas,” he said. “It’s a combination of visas. They’re visitors to the country. If they’re taking activities that are counter to our foreign, to our national interest, to our foreign policy, we’ll revoke the visa.”

He said he reviewed each case himself before signing off on actions that would be taken by immigration agents. Mr. Rubio said that a visa holder charged with a crime while in the United States should automatically lose their visa. He is also expelling permanent U.S. residents by stripping them of their green cards.

“My standard: If we knew this information about them before we gave them a visa, would we have allowed them in?” he said. “And if the answer is no, then we revoke the visa.”

Mr. Rubio declined to say how the cases arrived at his desk. “We’re not going to talk about the process by which we’re identifying it because obviously we’re looking for more people,” he said.

Mr. Rubio has been the most senior aide of President Trump involved in the contentious deportation efforts in communities across the United States. In mid-March, Mr. Rubio finalized a deal in which President Nayib Bukele of El Salvador agreed to accept U.S. deportation flights with more than 200 migrants, who were put into a prison there.

At least some of the migrants had fled to the United States from the repressive autocratic government of Venezuela and were not criminals, their lawyers say, but the United States failed to give them due process during their detention and deportation.

Mr. Rubio has also told the Homeland Security Department to detain students or recent graduates for deportation because of what he called their opposition to American foreign policy.

He told reporters on Thursday at a news conference in Georgetown, Guyana, that he had revoked the student visa of a Tufts University doctorate student and Fulbright scholar from Turkey. The student, Rumeysa Ozturk, was one of several authors of a student newspaper essay last year calling for university support of Palestinian rights and divestment from Israel.

After Mr. Rubio’s action, six people in black clothes and some wearing masks — presumably federal agents — seized her off a street outside her home in Somerville, Mass.

Mr. Rubio has also signed off on stripping the permanent residency status of two other students who were involved in pro-Palestinian protests at Columbia University: Mahmoud Khalil, born in Syria, and Yunseo Chung, born in South Korea.

Mr. Khalil, 30, is married to a U.S. citizen who was eight months pregnant when immigration agents seized him from their home in New York this month and took him to a detention center in Louisiana.

Ms. Chung, 21, has been in the United States since age 7 with her family.

Mr. Rubio has been named in separate lawsuits filed by Mr. Khalil and Ms. Chung, as well as other legal challenges to the visa and green card revocations and attempted deportations.

In Ms. Chung’s case, a federal judge ordered the Trump administration to halt its efforts to deport her.

Mr. Rubio has vigorously defended the actions when asked in recent days about them. He has avoided talking about details of each case but has described many of the people whose visas or green cards he has stripped as activists in movements whose participants have vandalized buildings, held disruptive rallies at universities and prevented other students from attending class.

“At some point I hope we run out because we’ve gotten rid of all of them,” he said in Guyana. “But we’re looking every day for these lunatics that are tearing things up.”

He added, “I encourage every country to do that, by the way, because I think it’s crazy to invite students into your country that are coming onto your campus and destabilizing it.”

Critics say Mr. Trump and Mr. Rubio’s actions are similar to those taken by authoritarian governments that seek to suppress free speech and assembly.

On Thursday, Mr. Rubio was asked twice whether under his rationale, Chinese Communist Party officials and authorities in Hong Kong had the right to deport foreign students involved in the 2019 pro-democracy protests in Hong Kong. As a senator from Florida, Mr. Rubio supported the protests through legislation, even though some of the protesters’ actions disrupted campuses and public life.

On the plane, Mr. Rubio said, “Well, every country in the world can deny visas to whoever they want. It’s that simple. That’s a fact. Whether we like it or not, they can deny visas.”