In volunteering as human shields, these and other visitors, mostly from the United States and Europe, are taking enormous risks and confronting the Israeli government with a public relations nightmare.
Last week, Israeli troops critically wounded a British man trying to save two children from Israeli fire. Days before, Karlsson's friend, American Brian Avery, was shot in the face in Jenin. Another American, Rachel Corrie, died March 16 trying to block an Israeli bulldozer in Gaza.
"If we're able to save just one Palestinian from Israeli gunfire, our work will have been worth all the risks and sacrifices," said Karlsson, 30, from Stockholm, his sandals covered in dust kicked up by the tank.
Israeli critics say the self-declared human shields represent a naive idealism that ignores the context that restrictions on Palestinians are meant to keep out the suicide bombers and other attackers who have killed about 750 Israelis in the past 2 1/2 years. The army also accuses one group of having sheltered a Palestinian wanted for planning attacks on Israelis.
Palestinians have long complained that Israeli troops use excessive force against civilians, and mounting casualties among foreigners are likely to draw renewed attention to the charge. In 30 months of fighting, more than 2,200 Palestinians have been killed most by Israeli troops.
The International Solidarity Movement, a group of Palestinians and foreigners based in the Palestinian areas, says more volunteers have been signing up since Corrie was killed. "Before Rachel's death, we had one or two people filling out registrations each day," said George Rishmawi, a founder of the group. "Now we get about eight per day."
The activists spend up to three months in the West Bank and Gaza, the areas the Palestinians claim for their future state. Many live on little more than pita sandwiches and cigarettes. Without a salary, money raised back home has to be stretched.
"Sometimes this situation feels pretty hopeless, but this area is ripe for change," said 40-year-old Kate Rafael from San Francisco who grew up in a Zionist family.
The group, founded several years ago by a group of Palestinians and Israelis gained attention last spring when Israeli troops invaded West Bank towns to root out militants after a spate of terror attacks. During Israel's operation, dozens of foreigners slipped past Israeli soldiers into two besieged hot spots Yasser Arafat (news - web sites)'s headquarters in Ramallah and the church on Christ's reputed birthplace in Bethlehem.
Israel has since deported or refused entry to dozens of foreign volunteers. About 50 foreigners, most from the United States, are currently in place.
Palestinians view them as saviors.
"If the world sees what the Israelis do to foreigners, they'll understand what we go through," said Mohammed Abdullah, 40, a Jenin shopkeeper.
The Israeli army insists it isn't deliberately killing innocents.
"We regret the loss of any civilian life whether it's Israeli, Palestinian or a Western national," said a military spokesman, Capt. Jacob Dallal, armed with diagrams that show blind spots on an armored D9 bulldozer, the type that the group said killed Corrie. The army denies the claim. "But these incidents could have been avoided by the protesters who have acted with reprehensible negligence."
Friday's shooting of 21-year-old Tom Hurndall from Manchester, England was witnessed by a photographer on assignment for The Associated Press.
A dozen activists walked toward Israeli tanks on the outskirts of the Rafah refugee camp, near the border with Egypt. They wanted to set up a tent in an attempt to block Israeli military incursions and were joined by several children, said photographer Khalil Hamra. The Israelis troops were firing toward a group of people and children, Hamra said. When Hurndall was tried to get two children out of the line of fire a tank opened fire and shot him in the head, Hamra said.
Hurndall remained comatose and hooked up to a respirator at an Israeli hospital Tuesday.
In Corrie's case, the army said the bulldozer with its limited visibility further reduced from the angle it was operating at didn't see her. Army video, however, shows a protester in front of the bulldozer. He said the video operator didn't tell the bulldozer operator Corrie was there but the army was considering placing closed circuit cameras inside the bulldozers.
Karlsson was with Avery when he was shot in the face April 5 in Jenin.
After hearing shots, the pair went downstairs from their apartment to investigate. Shortly after they walked onto the street, an armored personnel carrier rounded a corner and fired at Avery, tearing off part of his face, witnesses said. The army said a gunbattle with Palestinians was going on at the time and it was unclear who shot Avery. Witnesses from the group and Palestinians said there were no gunmen in the area when the troops fired at Avery.
The incident occurred days after Israeli troops raided the group's Jenin apartment and seized a member of the militant Islamic Jihad group who the army said had planned attacks against Israelis. The activists denied they had knowingly sheltered a militant. They said they let him in because they thought he was injured.
During a recent routine afternoon in Jenin, Karlsson tried to communicate in broken Arabic with a Palestinian family that had come to ask soldiers if they could pass along a road. The soldier barked at Karlsson in English, saying "What do you want!," then asked for the family's IDs.
It was a good day. No one was hurt and the Palestinians were allowed to pass.
"The daily humiliation that Palestinians have to go through ... is something I will never get used to," Karlsson says. "This is where my heart is."