Both Tsvangirai and Biti were briefly detained at Harare International Airport on Thursday, the MDC said.
"Operatives from the Central Intelligence Organisation (CIO) seized the MDC leadership's passports and briefly detained them, saying their names were 'on the list'. They did not reveal what so-called list was about," he said.
Tsvangirai told reporters earlier at the airport he was sure power-sharing talks with Zimbabwean president Robert Mugabe's government would resume.
The talks are mediated by Thabo Mbeki, the South African president.
He told reporters negotiations could not be judged on a deadlock over one issue, adding that there will always be people to break that deadlock.
Asked by reporters if he was still optimistic about a deal, Tsvangirai said: "Oh, yes, of course, we got our independence after how many talks? Hundreds and tens of meetings had been held."
Talks on power-sharing began last month after Mugabe's unopposed re-election in a June vote that was condemned around the world and boycotted by Tsvangirai because of attacks on his supporters by operatives of Zanu-PF, the governing party.
But three days of meetings in Harare failed to reach an overall deal.