South Korean police on Monday tried for hours to block a civic group from sending pro-democracy leaflets across the border by balloon after North Korea threatened the activists with a “merciless military strike,” but the South’s efforts ultimately failed.

Hundreds of police swarmed roads near the peninsula’s demilitarized zone, encircling the vehicles that were transporting activists to the planned launch site. After a 3½-hour standoff, the activists retreated, 200,000 leaflets still in the back of a van.

But hours later, after backtracking and eluding police, about 10 activists — compared with the 80 who started the day — made it to Ganghwa Island, not far from the border, where they released half of their leaflets.

“We wanted to show our strong intentions,” group leader Park Sang-hak said by phone.

Though it ultimately failed to stop the leaflet launch, the South’s attempt to intervene marked a sharp departure from recent years, when North Korean threats were either shrugged off or answered with daring defiance. Previous balloon launches by activists, which also sparked North Korean threats of military action, went off as planned.

Monday’s police-enforced de-escalation comes two months ahead of a tight election in the South to replace hard-line President Lee Myung-bak, and each of the three leading candidates have called for a more conciliatory approach to the North. Polls suggest that a majority of South Koreans want rapprochement, after several years in which Seoul cut off virtually all aid and Pyongyang responded with a pair of military strikes that caused multiple fatalities.

“The South Korean government must feel the need to manage the situation by not causing problems,” said Kim Heung-kyu, a political science professor at Sungshin Women’s University in Seoul. “They don’t want to give any excuses for provocations” ahead of the election, because “any North Korea-related problems now can be burdensome for the ruling party.”

A Defense Ministry spokesman said Monday that the police, not the government, were responsible for blocking the activists, and added that South Korea’s military was ready to retaliate if North Korea launched an attack. South Korean Defense Minister Kim Kwan-jin said Friday that the South would “completely destroy the origin of the attack” if Pyongyang followed through on its threat.

But the police chief in the city of Paju, near the demilitarized zone, said police were ordered to block the activists following a meeting that included the mayor, local police and the South Korean military.

“We thought there was a danger for civilians,” said the police chief, Kim Chang-sik.

Security analysts in Seoul say they have no reliable way to sift through the North’s regularly fiery rhetoric and separate the empty threats from the legitimate ones. Most prove to be little more than bluster. This year alone, the North laid out unusually specific pledges to “wipe out” Lee and his allies and carry out attacks on South Korean media outlets, even publishing the (incorrect) coordinates for several Seoul-based newspapers.

But not all threats are innocuous. Some experts here say the North telegraphed 2010 attacks on the South — the torpedoing of a warship, which has been attributed to the North and which killed 46, and the shelling of an island, which killed four — with boilerplate promises to exact “revenge.”

In the most recent case, the North’s threat — published Friday by the state-run news agency — was attributed to the Western Front Command, a frontline unit in the Korean People’s Army, or KPA. The North said the leaflets the activists planned to distribute, which are critical of new head of state Kim Jong Eun, were “slandering the dignity of [its] supreme leadership.” The North accused the South Korean military of engineering the event and ended its dispatch by warning residents in the border city to evacuate.

“The KPA never makes an empty talk,” the dispatch said.

The South Korean activist group was composed largely of defectors from the authoritarian North, who now routinely criticize its human rights abuses and leaders’ riches. Park, the group’s leader, says that Pyongyang considers him a traitor, and that he sometimes receives anonymous death threats. Seoul’s top spy agency last year said it arrested a North Korean agent who allegedly had been plotting to assassinate Park with a poison-tipped needle.

During Monday’s standoff with police, Park told reporters that he wanted to go ahead with the balloon drop and that he had gotten a permit for the protest nearly a month earlier.

“It’s not right to stop a legal protest because of North Korea,” Park said.

Yoonjung Seo contributed to this report.