It was the moment of conception of what would become the State of Israel — or it was the original sin in which Israel was conceived.

The Balfour Declaration, the pivotal, 67-word assurance by the British foreign secretary that promised support for “the establishment in Palestine of a national home for the Jewish people,” turned 100 on Thursday, meeting with tributes in Jerusalem and London, and tear gas in Bethlehem.

Demonstrators marched in nearly every major West Bank city. In Ramallah, thousands of flag-waving Palestinian Authority employees, Fatah youth guard members and schoolchildren, streamed to the mausoleum of the Palestinian leader Yasir Arafat and then to Al-Manarah Square, chanting “Balfour falls” and “We will not give up our right to exist.”