Tsvangirai is on trial for allegedly plotting to assassinate Mugabe and arrange a military coup ahead of presidential elections in March 2002, which the opposition leader lost to Mugabe.
State lawyer Joseph Musakwa told Judge Paddington Garwe yesterday that the prosecution wanted to include "discussion on transitional arrangements after the assassination as well as seeking military support after the elimination" in the charges.
Musakwa insisted the state was seeking only to make "minor corrections" to the charges, and that they would not prejudice Tsvangirai's defence.
But defence lawyers immediately opposed the application, arguing that the amendments were tantamount to putting the leader of the Movement for Democratic Change (MDC) on trial on different charges.
"It's a perfectly valid application because we are not coming up with any new charge," insisted Musakwa.
Tsvangirai was formally charged with treason just days after the presidential elections in March last year.
The opposition leader denies the charges, which carry the death penalty on conviction. He claims to have been set up by Ari Ben Menashe, a Canadian-based political lobbyist who Tsvangirai's lawyers say was secretly working for Mugabe's government.
Tsvangirai's counsel George Bizos, a veteran South African anti-apartheid lawyer, opposed the state application to modify the charges, saying the prosecution was not seeking an amendment "but a substitution".
"This is putting Mr Tsvangirai on trial for a different charge to the one he came to face," said Bizos.
He also dismissed the substance of the proposed amendment, saying: "There is no illegality in working towards a transitional government. It is not treason".
In a videotape of a meeting Tsvangirai held with Ben Menashe in Montreal in December 2001, which was secretly recorded by the lobbyist, Tsvangirai is heard to say he "would not be opposed to transitional arrangements" in Zimbabwe.
But Tsvangirai's lawyers yesterday argued that there was no evidence the MDC leader wanted to bring about a transitional arrangement "through unlawful means".
Unlike previous court appearances Tsvangirai yesterday was not flanked by MDC Secretary General Welshman Ncube, nor his shadow agriculture minister, Renson Gasela.
Charges against those two opposition officials, initially cited as co-conspirators with Tsvangirai, were dismissed in August due to lack of evidence.
The opposition leader had been expected to take the witness stand for the first time since the high-profile trial began.
But he once again sat in the dock, with his wife Susan and dozens of supporters crammed onto courtroom benches behind him. – Sapa-AFP.