Hakainde Hichilema was released from custody two weeks ago after being detained for allegedly failing to give way to President Edgar Lungu's motorcade.
"It is not because the police have done an investigation, it is because a member of the ruling party has instigated the police to arrest this citizen," Hichilema said.
"You cannot have a criminal justice system that operates like that."
The charges were widely seen as part of a crackdown on dissent by Lungu's Patriotic Front government.
Hichilema, leader of the United Party for National Development (UPND), was speaking at a joint press conference in Cape Town with South African opposition leader Mmusi Maimane.
Hichilema said he was "surprised" by his release as he was looking forward to being acquitted by the courts.
He was freed on August 16, when all charges were dropped hours before the start of his trial.
Hichilema, who narrowly lost last year's election and refuses to recognise Lungu as president, recounted details of his "brutal" arrest in April.
He said that more than "200 policemen, heavily armed with machine-guns" and assisted by German shepherd dogs surrounded his house and "poison-gassed" him, his wife and children for 10 hours.
Maimane, head of the Democratic Alliance party, was in May barred from entering Zambia after he landed in Lusaka airport to attend one of Hichilema's pre-trial court hearings.
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