Israeli officials had been due to hold a ceremony Thursday, marking the landmark in the year-old project, but the celebrations were cancelled in a bid to appease Washington, which has voiced displeasure over the fence.
Instead, foreign activists staged a demonstration in Qalqilya and hurled paint balls to the colours of the Palestinian flag against the wall, covering it with slogans in different languages.
Qalqilya is one of the spots where the barrier takes the shape of an eight-metre (25-foot) concrete wall, encircling the town and cutting it off from the rest of the West Bank.
"Tear down this wall! Free Palestine! Shame on you Bush! Berlin 1953! Roadmap to Apartheid!", were some of the hundreds of slogans the "graffiti commando" hastily splashed on the wall under tight army scrutiny.
"This wall is outrageous, it is aimed at solidifying occupation, through brutal means which resemble ethnic cleansing," said Canadian activist Mustafa Henaway, his mouth covered by a yellow bandana and green paint still dripping from his hands.
After hoisting a large banner reading "No Apartheid Wall" with two huge helium balloons, a member of the International Solidarity Movement (ISM) walked over to a gate in the wall, escorted by the Israeli army, to deliver a message to Israeli peace groups on the other side.
"I thanked them for their solidarity and called on them to do everything they can to end the occupation. The forces of evil built this wall, the forces of peace can take it down," said Noura Khoury, from San Francisco.
The Israeli defence ministry issued a statement announcing that the first, northern section of the barrier had been completed and insisting that the costly and controversial project was motivated by security rather than political concerns.
The statement said that 41 "agricultural gates" have been built along the fence to allow Palestinian farmers whose lands were left on the wrong side to cross, but the Palestinians complain the gates have yet to be opened.
According to Ali Zeid from the local farmers' union, some 56 hectares (140 acres) of farmland were destroyed during the construction of the fence and another 2,000 (5,000) annexed de facto by Israel.
"My guavas and apricots are falling from their trees and I can't pick them up to take them to the market because my fields are on the other side of the wall," he complained.
"The Israelis know exactly what they are doing, this is the best land in the West Bank and it has a lot of water. This wall is my prison, my coffin, they are slowly killing thousands of people," he charged.
According to the ISM and other rights groups, the 39,000-strong town, known as the West Bank's bread-basket, has seen 35 percent of its arable land and 33 percent of its water resources confiscated to make way for the wall.
Billed as a barrier between Israel and the West Bank, the security fence follows the 1967 Green Line between the two very loosely, dipping deep into Palestinian territory in a manner that will eventually clip off some 10 percent of the total.
While Israel insists the fence is vital to prevent infiltration by Palestinian militants, the Palestinians accuse Israel of using it to unilaterally determine the borders of a future Palestinian state.
Alongside the release of prisoners, the fence has become one of the most sensitive issues in the peace process and Washington has repeatedly voiced its displeasure.
"They only talk about releasing people from Hamas and Islamic Jihad. but what about people like us?", asked Qalqilya resident Munther Samleh. "I have been a prisoner in my own city for two years and I am not a gunman, I am just a farmer."
