A run-off presidential election between Tsvangirai and Mugabe, who leads the Zanu-PF party, is set for June 27.
In recent weeks, both sides have accused the other of intimidating voters in an attempt to influence the vote.
George Sibotshiwe, a spokesman for Tsvangirai, told Al Jazeera that Tsvangirai's arrest was an attempt to derail his election campaign.
"This is a deliberate strategy by Zanu-PF to disrupt Mr Tsvangirai's campaign and prevent him from having access to the Zimbabwean people," he said.
"He was told he could campaign but only at his risk. [The authorities] do not want him to campaign because they know he has popular support.
"Judging by what has happened so far, there is no way we can have a normal election.
"If election observers and peacekeepers can find their way to the ground as quickly as possible we can rescue this election. There is still a chance."
Also on Friday, aid groups banned from working in Zimbabwe by the country's government were told that they can resume work if they promise not to interfere in politics.
Bright Matonga, Zimbabwe's deputy information minister, said that all non-governmental organisations [NGOs] in Zimbabwe had been asked to re-register, a day after the ban came into force.
"[NGOs] were involved in political activities and behaving like political parties when they were supposed to complement government efforts," Matonga said.
|Mugabe is facing the stiffest test |
yet to his 28-year rule [AFP]
"As it appears that they veered from their normal work, we want them to clearly state what they intend to do, so that they will be bound by that."
He told Al Jazeera that the ban on NGOs was imposed as the government had doubts about the groups' impartiality.
"It is like having your friend here in a crisis - you ask for help, they provide the help and they sleep with your wife or your kid. You think that is a good friend?" he said.
But John Holmes, the UN chief relief co-ordinator, called on Zimbabwe to end the restrictions.
"This is a deplorable decision that vcomes at a critical humanitarian juncture for the people of Zimbabwe," he said.
"I strongly urge the government to reconsider and rescind this decision as soon as possible."
The suspension of all aid work on Thursday came about one week after some aid groups were banned for distributing food.
Those groups were accused of campaigning for the MDC in the presidential and parliamentary elections on March 29.
Matonga told Al Jazeera that groups were releasing aid to people on condition that they supported the opposition.
But Kieran Green of Care Canada, one of the groups ordered to halt operations in Zimbabwe, rejected the accusations.
James McGee, US ambassador to Zimbabwe, too said Mugabe's government is distributing its food aid to Zanu-PF supporters but not to those of the opposition.
"There is no question about that ... This is a very well orchestrated campaign," he said when asked by reporters over a video link in Washington if the Mugabe government was using its food aid as a weapon.
He said the group had a strict policy of not politicising aid and had "built a reputation on it for many many years".
The ban on aid groups came on the same day that three diplomatic vehicles - two American and one British - were attacked and several diplomats detained, according to officials from the two countries.
Gordon Johndroe, a US national security council spokesman, called Thursday's alleged attack "outrageous" and "completely unacceptable".
The UK and US later said that their diplomats had been freed.
The US government took complaints about the incident to the UN Security Council.
The government in Harare said that the diplomats had been addressing a gathering at the home of an opposition party activist.