* Says Mugabe orders probe into killing of MDC aide's son
* Mugabe, Tsvangirai to face off in presidential vote
By MacDonald Dzirutwe
HEADLANDS, Zimbabwe, Feb 28 (Reuters) - Zimbabwe's premier said on Thursday President Robert Mugabe's party was losing its grip and resorting to violence after the young son of a local party official was burnt to death by suspected supporters of the elderly president.
Mugabe, 89, in power since 1980, and his longtime foe, Prime Minister Morgan Tsvangirai, were forced into a power-sharing government in 2009 and will resume their rivalry in elections expected around July.
Christpowers Maisiri, 12, whose father is in Tsvangirai's Movement for Democratic Change (MDC) party, was set on fire while sleeping in a hut with his brothers last weekend in Headlands district, 170 km (110 miles) east of Harare.
The MDC quickly blamed Mugabe's ZANU-PF party, saying the alleged killers were after the boy's father, Shepherd. ZANU-PF denied killing the boy and accused the MDC of trying to fan pre-election tensions in the southern African state.
"ZANU-PF is under siege. They are in a corner and this is a desperate act from a party that is losing power," Tsvangirai said while addressing mourners and supporters attending the burial in Headlands, a tobacco-growing district.
Fighting back tears, Shepherd Maisiri said he had been subjected to violence and intimidation from ZANU-PF supporters since 2000. His son had been born in the bush because his parents had to flee from opponents, he said.
Tsvangirai said he had shown Mugabe pictures of the charred remains of the boy during a cabinet meeting on Tuesday and that the president had "shrunk" in disbelief and ordered a full investigation.
Tsvangirai accuses Mugabe of using violence to cling to power. The former trade union leader was forced to quit a presidential run-off race in 2008 after 200 of his supporters died in political violence blamed on ZANU-PF members.
"We are hurt but not intimidated. This has to end, starting with the arrest of the people who committed this heinous crime," said Tsvangirai. He said he could not vouch for Mugabe's sincerity when calling for peaceful elections. (Reporting by MacDonald Dzirutwe)