Tendai Biti, spokesman for Tsvangirai's Movement for Democratic Change party told the BBC about 20 other party members were also taken away in the raid on party headquarters.
"There's a wall of riot police so you can't actually see what's happening," Biti said.
At the news conference, Tsvangirai was expected to speak about recent alleged abductions of party members by the government, Biti said.
Earlier this month, Tsvangirai and scores of anti-government activists were arrested and many were beaten by police at a demonstration President Robert Mugabe had banned.
This week in Tanzania, southern African leaders will meet to discuss Zimbabwe's decline and Mugabe is expected to attend on Thursday.
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