It was unclear Saturday if Park was in North Korean custody. Illegal entry into the country is punishable by up to three years in prison. The communist regime held two American journalists for five months earlier this year before freeing them during a visit by former President Bill Clinton.
Park is a missionary from Tucson, Arizona, according to the activist, who works for Pax Koreana, a conservative Seoul-based group that calls for North Korea to improve its human rights record.
"I am an American citizen. I brought God's love. God loves you and God bless you," Park was quoted by two activists as shouting in Korean as he crossed the North Korean border, according to the activist who spoke to The Associated Press.
He said Park was last seen by the two other activists, who saw him enter North Korea's northeastern city of Hoeryong from the poorly guarded border late Friday afternoon. He added that the crossing was videotaped and the footage would be released Sunday.
North Korea holds some 154,000 political prisoners in six large camps across the country, according South Korean government estimates. The North has long been regarded as having one of the world's worst human rights records, but it rejects outside criticism and denies the existence of prison camps.
North Korean state media did not mention any illegal crossing. The country's criminal code punishes illegal entry with up to three years in prison.
Park carried a letter to Kim calling for major changes in how the country is operated, according to Pax Koreana.
"Please open your borders so that we may bring food, provisions, medicine, necessities, and assistance to those who are struggling to survive," said the letter, according to a copy posted on Pax Koreana's Web site. "Please close down all concentration camps and release all political prisoners today."
The activist said that Park also carried a separate written appeal calling for Kim to immediately step down, noting alleged starvation, torture and deaths in North Korean political prison camps. The second letter was addressed to the leaders of South Korea, China, the U.S., Japan and the United Nations.
North Korea is expected to react strongly because Park raised the issue of its political system, said Koh Yu-hwan, a professor at Seoul's Dongguk University.
Demanding Kim step down is "a kind of hostile act" and "the North won't likely compromise on such issue," Koh said, predicting it will take time to resolve.
Kim wields absolute power in the communist state of 24 million people. Any acts seen as hostile to him and his leadership carry harsh punishment, said Choi Eun-suk, a professor on North Korean legal affairs at the Institute for Far Eastern Studies at Kyungnam University in Seoul.
The U.S. Embassy in Beijing said it is looking into Park's case, but it had no details.
"His fate to us is unknown," said embassy spokeswoman Susan Stevenson. She said a charitable organization, which did not identify, had notified the State Department in Washington of Park's actions.
The activist said Park came to South Korea in July and stayed there until leaving for China earlier this week to enter the North.
"I would not go to North Korea to live. Even if I die, world leaders should really repent for keeping silence" on North Korea, Park said in Seoul before leaving for China, the activist said.
The activist said Pax Koreana is affiliated with another organization called Freedom and Life For All North Koreans, which is a coalition of advocacy groups for North Korean human rights. Park is a member of the broader group, he said. The coalition and other activist groups plan to hold rallies in New York, Tokyo, Seoul and other cities from Sunday to Thursday.
In August, North Korea released two U.S. journalists it had sentenced to 12 years of hard labor for trespassing and "hostile acts." Their release came amid a trip to Pyongyang by former President Clinton aimed at winning their freedom.
American journalists Laura Ling and Euna Lee were captured by North Korean guards near the Tumen River in March while reporting a story on North Korean defectors.
Park's reported entry comes weeks after North Korea held one-on-one talks with the United States and signaled its willingness to return to international negotiations on ending its nuclear weapons programs. Pyongyang said earlier this month it would try to resolve remaining differences with Washington.