Freetown - A key third party in Sierra Leone announced on Wednesday it would not ask supporters to back either contender in a March 27 presidential run-off, after police warned violence has plagued campaigning.With the margin between the opposition Sierra Leone Peoples' Party (SLPP) and ruling All Peoples' Congress (APC) on a knife-edge, tensions are high in the West African nation.The National Grand Coalition (NGC), which took 6.9% of votes in the March 7 first round with an issues-focused platform, said it would support neither and had "grave misgivings" about the process."The message from the overwhelming majority of NGC members and supporters is clear: NGC should not endorse either APC or SLPP or enter into a coalition arrangement with either party," it said in a statement. "Rather, the NGC should concentrate on providing an effective opposition on behalf of the people of Sierra Leone," it added.The party, popular with educated, urban youth, meanwhile warned against the "increasing emergence of tribalism in the politics of the country", which it said was "leading our nation on a path to self-destruction."Police said in a statement on Monday that nine separate incidents including violent attacks and stone throwing had been recorded across the country in the preceding week, all linked to the election."There is a growing political intolerance, political motivated and targeted attacks on people, all of which together give a feeling of deteriorating security situation," the statement said.Sierra Leone, battered by a horrific 1991-2002 civil war, is sharply divided along regional lines that overlap with ethnicity at election time.The APC broadly relies on the Temne and Limba people in its northern strongholds, while the SLPP is more popular in the south with the Mende ethnic group.KEEP UPDATED on the latest news from around the continent by subscribing to our FREE newsletter, Hello Africa.FOLLOW News24 Africa on Twitter and Facebook. Opposition leader Julius Maada Bio, from the Sierra Leone People's Party (SLPP), took 43.3% of votes in the first round, while Samura Kamara of the incumbent All Peoples Congress (APC) took 42.7%.The African Union (AU) observer mission in the country said on Tuesday it had "learned with dismay the worrisome prevalence of election-related violence which is increasingly become wide-spread across the country."Sierra Leone's Supreme Court is meanwhile considering a request for an injunction lodged by a lawyer with links to the ruling APC which would seek to delay the vote. A decision on whether to take further action or throw out the request is expected on Friday.