Zimbabwe’s police accuse Mr Biti of inciting violence following last month’s election.
Zambian Foreign Minister Joe Malanji told the BBC that Mr Biti’s grounds for asylum were weak.
He was being kept in “safe custody” until he returned to Zimbabwe, the minister said.
Earlier, his lawyer said his client had been detained at the border with Zambia by the Zimbabwean authorities.
An alleged Zambian police report circulating on social media states that Zimbabwean officials tried to detain Mr Biti after he had crossed into Zambia.
The opposition politician shouted out for help and about 300 Zimbabwean travellers blocked their government’s security officers from making an arrest, the report says.
Zambian officials then intervened and threatened to arrest the Zimbabwean officers for “executing their mandate on Zambian soil”.
Zambia’s foreign minister told the BBC that Mr Biti’s grounds for asylum were “not meritorious”.