North Korea's military warned last week that it would strike if the South Korean activists carried through with their plan, and South Korea pledged to retaliate if it was attacked.
South Korean police, citing security concerns, had sent hundreds of officers Monday to seal off roads and prevent the activists and other people from gathering at an announced launch site near the border. Residents in the area were also asked to evacuate to underground facilities, according to local official Kim Jin-a.
Later in the day, some of the activists, mostly North Korean defectors, moved to another site near the border that was not guarded by police and launched the balloons. They had described the police response as a surrender.
South Korea's Defense Ministry said there were no suspicious activities from North Korea's military.
Before acting Monday, the South Korean government had implored activists to stop their campaign but had cited freedom of speech in not making further attempts to intervene. South Korean activists have sent leaflets across the border in the past, and North Korea has issued similar threats to attack without following through.
Seoul's Yonhap news agency reported Monday that the ban on entering the
Defense Ministry spokesman Kim Min-seok told reporters North Korea was believed to have acted in line with carrying out its threat. He declined to elaborate on the North's army movement as that was confidential military information.
He said South Korea had bolstered its military readiness following the North's threat and would "strongly" retaliate if attacked.
The activists said they floated balloons carrying about 120,000 leaflets critical of North Korea's young leader Kim Jong Un and his country's alleged human rights abuses. They said they wanted to let North Korean people know the true nature of their country.
"We could not delay our plans to send anti-North Korea leaflets because it is our love toward our northern brothers," the activists wrote in a statement posted on the website of Seoul-based Free North Korea Radio, one of civic organizations involved in the leafleting.
Lead activist Park Sang-hak had said the ban on entering the border area was tantamount to yielding to Pyongyang's threat.
"It's surrender. It's clearly surrender," he said.
The top U.S. envoy on North Korea urged Pyongyang to stop issuing destabilizing threats.
"It is grossly disproportionate to have threatened to respond to balloons with bombs," Glyn Davies told reporters in Beijing after meeting with Chinese officials.
China, the North's main ally and biggest aid source, welcomed South Korean efforts to quash the balloon-flying and urged all parties to exercise restraint.
"As a close neighbor to the peninsula, China supports dialogue and discussions between North Korea and South Korea in resolving relevant issues, opposes any action which may heighten tension, and firmly opposes military conflicts on the peninsula," Chinese Foreign Ministry spokesman Hong Lei said at a daily media briefing. "We hope the parties involved will stay calm and restrained."
Ties between the rival Koreas were badly strained after two deadly attacks blamed on North Korea killed 50 South Koreans in 2010.
The Korean Peninsula officially remains at war because an armed conflict in the 1950s ended with an armistice, not a peace treaty.
The latest flare-up in tensions comes as almost all the regional players are consumed with domestic politics. Elections are being held or are expected soon in South Korea, the United States and Japan, while China's Communist Party is in the midst of transferring power to a younger generation of leaders.
Associated Press researcher Zhao Liang contributed to this report from Beijing.