Lawyers for jailed Zimbabwean opposition leader Mr Morgan Tsvangirai have urged a judge to free him on bail.
They claim treason charges are being used to silence President Robert Mugabe's chief political foe.
Mr Tsvangirai was arrested last Friday after five days of protests and work boycotts organised by his Movement for Democratic Change that paralysed the country.
Defence lawyer Mr George Bizos said the government was playing "fast and loose" with the law and urged Justice Susan Mavangira to release his client - who lost to Mr Mugabe in what the opposition said was a rigged presidential election last year.
"By keeping political opponents silent, the difficulties with which the country is faced are not solved," he said.
Mr Tsvangirai has emerged as the biggest threat to Mr Mugabe's 23 years in power as Zimbabwe plunges deeper into political and economic crisis.
The opposition, which accuses Mr Mugabe of repressive rule and ruining the economy, described last week's protests as a "final push" against the president.
Mr Bizos, who defended South African anti-apartheid icon Nelson Mandela in his treason trial four decades ago, said denying Mr Tsvangirai bail would silence a man who won more than a million votes in the disputed 2002 presidential election.
A lower court ordered Mr Tsvangirai on Tuesday to be held in detention until July 10th unless granted bail by the High Court.
Mr Tsvangirai already faces a possible death sentence in a separate treason trial for an alleged plot to assassinate President Mugabe in 2002.