kinshasa police fire tear gas disperse supporters democratic republic congo opposition chief sunday gather capital amid high tensions election delay keep president joseph kabila power reuters witness say felix tshisekedi head opposition coalition due fly back capital kinshasa sunday prolong absence abroad around dozen supporters gather family residences morning although sign violence tshisekedi intend call civil disobedience campaign order pressure kabila whose mandate expire last year step aside congolese authorities ban coalition plan meet sunday cite risk violence central african country 70 million people police fire tear gas congo opposition leader 's supporters