* Military on collision course with Mugabe and his wife
* Mugabe chairing a weekly cabinet meeting in the capital
* Military has anchored Mugabe's 37-year-old rule
Armoured vehicles were seen heading towards Harare but the streets of the Zimbabwe capital remained calm, a day after the armed forces chief said he was prepared to "step in" to end a purge of supporters of a vice president sacked last week.
A Reuters witness saw two tanks parked beside the main road from Harare to Chinhoyi, about 20 km (14 miles) from the city. One, which was pointed in the direction of the capital, had come off its tracks.
Business continued normally inside the capital and there was no sign of a major military presence on the streets. Hours after the tanks were spotted, state media carried no extraordinary reports. Government officials could not be reached for comment.
Witnesses said they saw four armoured vehicles turn before reaching Harare, heading towards the Presidential Guard compound in a suburb called Dzivarasekwa on the outskirts of Harare.
"There were about four tanks and they turned right here, you can see markings on the road," one witness on the Chinhoyi highway said pointing to a road that links up to the Presidential Guard compound that houses the battalion that protects President Robert Mugabe.
Mugabe, the only leader Zimbabwe has known in 37 years of independence, chaired a weekly cabinet meeting in the capital.
In an unprecedented step, the head of the armed forces, Constantino Chiwenga, openly threatened to intervene in politics on Monday, a week after Mugabe fired Vice President Emerson Mnangagwa, long seen as 93-year-old Mugabe's likely successor.
Mnangagwa, a veteran of Zimbabwe's 1970s liberation wars, was popular with the military, which viewed his removal as part of a purge of independence-era figures to pave the way for Mugabe to hand power to his wife Grace, 52.
"We must remind those behind the current treacherous shenanigans that, when it comes to matters of protecting our revolution, the military will not hesitate to step in," Chiwenga said in a statement read to reporters at a news conference packed with top brass on Monday.
Grace Mugabe has developed a strong following in the powerful youth wing of the ruling party. Her rise has brought her into conflict with the independence-era war veterans, who once enjoyed a privileged role in the ruling party under Mugabe, but who have increasingly been banished from senior government and party roles in recent years.
Neither the president nor his wife responded immediately to the general's remarks, but on Tuesday the head of ZANU-PF's youth wing accused the army chief of subverting the constitution.
"Defending the revolution and our leader and president is an ideal we live for and if need be it is a principle we are prepared to die for," Kudzai Chipanga, who leads the ZANU-PF Youth League, said at the party's headquarters in Harare.
The rising political tension in the southern African country comes at a time when it is struggling to pay for imports due to a dollar crunch, which has also caused acute cash shortages.
Zimbabwe's state media refrained from publishing Chiwenga's statement. The Herald newspaper, which had initially posted some of Chiwenga's comments on its official Twitter page on Monday, deleted the posts without explanation.
A senior South African diplomat said Pretoria had scrambled its officials in Harare to try to find out what was going on, but at the moment they had little conclusive information.
Martin Rupiya, an expert on Zimbabwe military affairs at the University of South Africa in Pretoria, said the army appeared to be putting the squeeze on Mugabe.
"There’s a rupture between the executive and the armed forces," Rupiya said.
Alex Magaisa, a British-based Zimbabwean academic said it was premature to talk about a coup.
"A military coup is the nuclear option. A coup would be a very hard sell at home and in the international community. They will want to avoid that," Magaisa said.