The arrest was Tsvangirai's second in three days, The Times of London reported. On Thursday, a group of British and U.S. diplomats on a trip to investigate campaign violence were detained.
Tsvangirai was halted at a road block. He was held for two hours at a police station in Esigodini and then ordered to stay away from a rally in Now Mine.
According to official election results, Tsvangirai defeated Mugabe in the March 29 election but failed to win the absolute majority required to avoid a runoff. His party, the Movement for Democratic Change, also won more parliamentary seats than Mugabe's Zanu-PF.
MDC Secretary General Tendai Biti denounced the detentions at a World Economic Forum for Africa meeting in South Africa.
"It is almost as if the regime is sending out a message to the region, to the international community that it doesn't care, that it has no respect for life, it has no respect for the rule of law," Biti said. "The regime is increasing the decibels of insanity."