The article accused Mugabe of destroying the Zimbabwean economy and his security forces of abuses.
The editor of The Standard newspaper was arrested last month over the piece written by Mutambara.
Mutambara recently pledged to support Morgan Tsvangirai, leader of the main MDC group, in the run-off poll against Mugabe.
Police also arrested a lawyer for Tsvangirai and the opposition says at least 50 of its activists have been killed, hundreds injured and at least 25,000 displaced in attacks in the run up to the country's presidential run-off on June 27.
The ruling Zanu-PF says its members have also been targeted.
Meanwhile, Mugabe arrived in Rome on Sunday for a summit of the UN's Food and Agriculture Organisation as his attendance was described as "obscene" by Australia's foreign minister.
Steven Smith said on Monday that it was not right for Mugabe to be at the summit.
"This is the person who has presided over the starvation of his people ... [He] has used food aid in a politically motivated way," he said.
|"Robert Mugabe turning up to a conference dealing with food security or food issues is, in my view, frankly obscene"|
Steven Smith, Australia's foreign minister
"So Robert Mugabe turning up to a conference dealing with food security or food issues is, in my view, frankly obscene."
Gordon Brown, the British prime minister, also condemned Mugabe's presence.
"We think it's particularly unfortunate that he has decided to attend this meeting given what he has done in relation to contributing to difficulties on food supply in Zimbabwe," a spokesman said.
The 84-year-old is usually subject to a travel ban to the European Union because of sanctions imposed after he allegedly rigged his re-election in 2002, but is able to attend UN forums.
Zimbabwe was once considered a self-sufficient country, able to feed its own people. But the country is now facing acute food shortages after a land reform programme and the collapse of the farming sector.
Thousands of white-owned farms were seized and handed over to black Zimbabweans.
Mugabe's critics say that that many of the best farms ended up in the hands of his close supporters.
After elections on March 29, Mugabe's Zanu-PF lost its majority in parliament for the first time in 28 years and Tsvangirai won the presidential election, but without the overall majority required for an outright win.
A delay in the release of election results triggered a wave of violence across the country, particularly in rural areas.
Last week, six veterans of the liberation war against white rule in the 1970s and officials from Zanu-PF were injured following attacks by unidentified people, according to the state-run Herald newspaper.
Levy Sibanda, the deputy police commissioner-general, said: "So far, more than 70 suspects have been arrested in connection with the cases and are still in police custody.
"We are here to assess the situation and to find ways on how best peace can prevail in this area. We are solving the problem to ensure that people live in peace."