The EC Regulation 261/2004 can be found here.
While the EC Regulation 261/2004 only related to flights departing the European Union and EU airlines flying to the European Union from a third country, the Montreal Convention (formally, the Convention for the Unification of Certain Rules for International Carriage by Air) is an international, multilateral treaty adopted by a diplomatic meeting of ICAO member states in 1999.
The Montreal Convention 1999 has re-established uniformity and predictability of rules relating to the international carriage of passengers, baggage and cargo.
The protection of passengers in the Montreal Convention related to damages caused by the delay of a flight and the liability of air carriers in the event of death or injury of passengers.
The Montreal Convention concerns individual damage caused by a long delay while the EC Regulation 261/2004 relates to standardized and immediate compensatory measures to all passengers regarding the loss of time caused by the delay.