Interviewed by the Associated Press in neighboring South Africa, Tsvangirai called the treason charges "outrageous". He said his party, the Movement for Democratic Change (MDC), was formed with a commitment to "democratic change" in Zimbabwe.
The accusation came as the government continued a campaign of arrests, assaults and other intimidation designed to suppress political dissent following a March 29 vote that President Robert Mugabe is widely believed to have lost.
Zimbabweans have been waiting nearly three weeks for results of the presidential vote as riot police and security forces have deployed across the country in a show of force.
Tsvangirai also said Thursday that Mugabe's continued abuses mean he may have to face justice.
In the past, Tsvangirai has said he would not be interested in a "witch hunt" against Mugabe because it would distract a new government from Zimbabwe's economic and political crises, but he said he is now no longer so ready to "forgive" Mugabe.
Independent tallies suggest Tsvangirai won, but not with enough votes to avoid a runoff. The electoral commission plans a re-count of presidential votes on Saturday, saying it is verifying ballots and investigating anomalies.
The opposition says they won outright, and accuse Mugabe of engineering a delay to secure his 28-year grip on power.
South Africa's opposition Democratic Alliance said South African parliamentarians had been invited to observe the recount but that it first is seeking assurances - including that ballot boxes had not been tampered with and that the recount would immediately produce an announcement of results.
Tsvangirai's party has failed in attempts to force the release of results through the courts and through appeals to regional leaders. The party has been reluctant to agree to a runoff, arguing that a second round would be rigged by Mugabe's cronies.
To support the treason charges, the state-run Herald newspaper quoted Thursday from alleged correspondence between British Prime Minister Gordon Brown and Tsvangirai. Tsvangirai's Movement for Democratic Change has called the document - cited repeatedly this week by the Mugabe administration - a forgery.
The Herald said Brown pledged in an April 9 letter to Tsvangirai that he would "make sure that a solution to the ongoing crisis in Zimbabwe is reached and your electoral success is respected."
"Tsvangirai along with Brown are seeking an illegal regime change in Zimbabwe, and on the part of Tsvangirai, this is treasonous," The Herald quoted outgoing Justice Minister Patrick Chinamasa as saying. Chinamasa lost his seat in the elections in which Mugabe's ruling party lost control of Parliament for the first time.
Officials from former colonizer Britain would not immediately comment on the letter or its authenticity.
Tsvangirai has been charged with treason before. In 2003, he was acquitted of charges in an alleged plot to assassinate Mugabe. Tsvangirai called the 18-month trial an attempt to frame him and fellow opposition leaders.
At the United Nations on Wednesday, the United States and Britain backed a suggestion from U.N. Secretary General Ban Ki-moon to send U.N. observers to monitor any presidential runoff.
U.S. Secretary of State Condoleezza Rice on Thursday called Mugabe's recent rule an abomination and urged his country to release the election results. Rice said it must be decided in Zimbabwe whether Mugabe steps down. But, she added, Zimbabwe "really needs to move on and get on with its future."
The Herald indicated that was unlikely. It quoted Zimbabwe's U.N. Ambassador Boniface Chidyausiku as saying that "for the UN to come we are saying it should first be invited."
Zimbabwe refused to allow Western observers to monitor last month's elections, inviting only "friendly" countries including a Southern African Development Community team led by a junior minister from Angola - a country that has not had elections since 1992.
Zimbabwe's government also warned Thursday that it will pull the licenses of any transport workers who heed an opposition call to strike for the release of election results.
With Zimbabwe's economy devastated by soaring inflation and 80 percent unemployment, the opposition has had difficulty getting the few Zimbabweans with jobs to join the nationwide strike.
But The Herald quoted Transport Minister Chris Mushowe as saying some public buses had stopped running, "deliberately withdrawing their services since Monday." Mushowe said this violated terms of transport licenses.