Yassin was assassinated in a targeted Israeli airstrike in 2004, after scores of suicide bombings and rocket attacks by the radical Islamist movement.
Israel unilaterally withdrew from the Gaza Strip in 2005. Hamas won Palestinian parliamentary elections that same year, and seized sole control of the coastal enclave 18 months later, by violently overpowering the security forces of moderate Palestinian President Mahmoud Abbas.
Hamas' de-facto prime minister in Gaza and Abbas' rival, Ismail Haniya, used the anniversary to call on Abbas to withdraw from peace negotiations with Israel, which he called "absurd."
The turn-out in Gaza was a clear "no to negotiations, no to compromise and yes to resistance," Haniya told the crowd, waving green Hamas flags and wearing green caps and headbands.
Abbas' peace talks with Israel, which faces an April deadline, should not be extended "under any reason or pretext," he said.
Unlike Abbas, Hamas has refused to renounce violence, recognize Israel's right to exist and endorse past interim peace deals calling for a two-state solution to the conflict. It advocates an Islamic state in what is now Israel, the West Bank and the Gaza Strip.