President Robert Mugabe of Zimbabwe says some ruling party officials have been plotting to oust him.

The 92-year-old Mugabe warned of threats to his long rule during a speech Friday at the opening of a ruling party conference in the southeast town of Masvingo.

The ZANU-PF party is expected to endorse Mugabe as its candidate for 2018 elections. He has been in power since independence from white minority rule in 1980.

Mugabe says some ruling party members have been punished for opposing his authority, and is warning others to stop using Twitter to criticize each other as a succession battle intensifies.

Opposition groups have organized periodic protests against Mugabe, saying he is too old and accusing him of mismanaging Zimbabwe’s weak economy.