“We will keep on fighting,” Tendai Biti told reporters, before being granted bail of $5,000 (£3,885) and told to surrender his passport.
The UN said it was gravely concerned that Biti, a former finance minister and senior figure in the Movement for Democratic Change, was forcibly returned to Zimbabwe, where police took him into custody before his court appearance.
“Refoulement or forcibly returning refugees and asylum seekers to their country of origin is a serious violation of international refugee law,” the UN said.
Police in Zimbabwe had spent days looking for Biti, who they claim incited and organised violence after last week’s disputed national election, which was won by President Emmerson Mnangagwa, the leader of the ruling Zanu-PF party.
Eight other MDC leaders were also being sought by law enforcement agencies.
“The facade has come off. They have realised they can only govern by coercion. I am deeply concerned about Tendai,” said David Coltart, a senior MDC official.
But hopes that the relatively peaceful campaign and voting would herald a political and economic transformation of the impoverished former British colony have been dashed by violence and alleged human rights abuses since the election.
Six people died last week when soldiers opened fire on MDC protesters in central Harare.
According to dozens of testimonies gathered by the Guardian, security forces have since harassed hundreds of MDC leaders and activists, arbitrarily detaining and beating scores of people.
Last week the homes of opposition leaders and activists were surrounded by unidentified armed groups during the night and gangs shouting pro-government slogans forced their way into the properties.
At the weekend, soldiers moved through neighbourhoods of Harare and surrounding towns, targeting opposition supporters, smashing property and assaulting dozens of people.
In the past 72 hours, dozens of independent media activists have gone into hiding, along with several NGO workers fearful of detention.
Nelson Chamisa, the leader of the MDC, has dismissed the election as “fraudulent and illegitimate”.
Biti’s Zambian lawyer Gilbert Phiri said his client, 52, was handed over to Zimbabwean law enforcement officers on Thursday morning, despite a court order preventing his forcible return.
At a packed court hearing on Thursday afternoon, prosecutors accused Biti of egging on protesters on 1 August, urging them to pull down a Mnangagwa campaign poster while telling them: “Remove that poster, we don’t want thieves in this country.”
The protesters went on to burn the poster while damaging property at Zanu-PF’s offices in the capital.
Explaining why Zambia refused Biti asylum, Dora Siliya, the Zambian information minister, said: “One can only seek asylum if their country of origin is under massive violence and there is total breakdown of law and order and, indeed, the lives of citizens there are threatened. In this case, it was concluded that did not apply.”
Zimbabwean lawyers were prevented from following the police vehicles carrying Biti back from the border and lost track of his whereabouts until being told around noon that he might be in Harare central police station.
British diplomats have spoken to Zambian and Zimbabwean authorities to express concern.
“We continue to follow Tendai Biti’s case very closely. We and others are seeking clear assurances from the Zimbabwean authorities that his safety will be guaranteed and constitutional rights fully respected,” a statement issued by the British embassy in Harare said.
Biti had been in hiding since last week and feared for his life, his lawyer Nqobizitha Mlilo said.
In a brief communication with the Guardian after going underground, Biti described the authorities as “murderers and fascists”.
Phiri said the Zambian high court had on Wednesday night issued an order to stop Biti’s deportation but Zambian immigration and police had refused to accept the court papers.
Biti had sought asylum by attempting to enter Zambia through the Chirundu border post, 220 miles (350km) north of Harare, but his application was rejected.
He was then moved to a school near another border crossing in Kariba before being handed over to Zimbabwean police, Phiri said.
“We tried to serve the court order on the immigration officers who were keeping him at a school under police protection but they refused to take it,” the lawyer added.
Authorities in Zimbabwe need international legitimacy to obtain the multibillion-dollar bailouts required to avoid economic collapse. The violence is a serious setback and some say the crackdown suggests splits within the ruling elite.