HARARE, April 3 (Reuters) - Zimbabwean President Robert Mugabe, facing the biggest crisis of his rule, will chair a meeting of the politburo of his ruling party on Friday, ZANU-PF sources said on Thursday.

Earlier a senior party official said the politburo would discuss Zimbabwe's elections, in which ZANU-PF lost control of the lower house of parliament for the first time in almost three decades.

There is uncertainty over whether Mugabe would stand in a runoff vote against opposition leader Morgan Tsvangirai.