By Nick Carey and Edward McAllister FERGUSON Mo. (Reuters) - Demonstrators in Ferguson, Missouri, on Saturday marked two weeks since a white police officer shot dead an unarmed black teenager, while authorities reported no arrests overnight among the pockets of protesters who marched and chanted in intense heat. The St. Louis suburb has had three consecutive relatively calm nights after daily turmoil since Michael Brown, 18, was shot by Ferguson officer Darren Wilson on Aug. 9. The St. Louis County NAACP chapter plans to march in the afternoon and supporters of the officer plan a fundraiser on Saturday and Sunday at a bar in a nearby community. Groups of protesters numbering about 20 to 30 each marched up and down the streets.