The swearing-in of the parliament which was elected last month paves the way for the Islamist party to form a government.
It's on a potential collision course with Abbas.
Hamas has up to five weeks to form a government.
If that government refuses to heed Abbas' demand to recognise past peace accords and accept negotiations, then Abbas, as president of the Palestinian Authority and head of the Palestine Liberation Organisation, has the right to dismiss it.
It also faces a boycott by major powers unless it renounces violence and its vow to destroy Israel.
Israel has threatened to close the border with Gaza and impose harsh economic sanctions on a Hamas-led Palestinian government.
It has also been considering a new military deployment.