Palestinian President Mahmood Abbas described his death as “a barbaric act, which we cannot be silent about or accept”.
Mr Abu Ein was part of a 100-strong group of foreign and Palestinian activists en route to plant olive trees and protest near an Israeli settlement when they were stopped at an improvised checkpoint.
A group of around 15 Israeli troops fired tear gas and began scuffling with them, according to eyewitnesses. During the incident Mr Abu Ein was pushed and shoved, and struck by a hand to the neck, according to a Reuters photographer at the scene.
Mr Abu Ein was rushed by ambulance from the scene to hospital in Ramallah, but died on the way. Shops in the city closed in protest at his death and youths threw stones at Israeli soldiers guarding a Jewish settlement outside the city.
Israeli army said it was investigating the incident.
Mr Abu Ein, who was in his 50s, headed the government-run protest organisation called the committee to resist Settlements and the wall, which was behind today’s tree planting action.