HARARE (Reuters) - Ousted Zimbabwean vice president Emmerson Mnangagwa added his voice on Tuesday to those demanding 93-year-old President Mugabe resign, saying he needed to heed the  clarion call  of his people and step down. Mnangagwa, who said he fled Zimbabwe because of a threat to his life after being purged from the ruling party, said he had been in contact with Mugabe and invited to return but would not do so until his personal security could be guaranteed.  I told the President that I would not return home now until I am satisfied of my personal security, because of the manner and treatment given to me upon being fired,  he said in a statement. 