Former Tanzanian MP Godbless Lema spent Sunday night in a police station in Kenya moments after irregularly crossing the border, according to Kenyan media outlets.
There has been a crackdown on opposition leaders following President John Magufuli's re-election in a landslide victory.
Mr Lema was arrested while being driven by his lawyer to Kenya's capital, Nairobi. He was accompanied by his wife and children.
Kenyan police caught up with them at a town located about 45km (28 miles) from the border after being alerted by their Tanzanian counterparts, the Standard newspaper reports.
The former MP had reportedly declined to hand over his passport to Tanzanian immigration officials.
His lawyer, George Luchiri Wajackoyah, is quoted by the newspaper as saying he wanted to present the politician to the United Nations Human Rights Commission office in Nairobi.
“Right now, I am with my wife Neema, my 14-year-old son Allbless, Brilliance (daughter) my last born son Terrence. I do not know what tomorrow holds. I am now looking for asylum,” Mr Lema is quoted as saying by the newspaper.
Mr Lema was among three opposition leaders who were released last week after being arrested for calling for demonstrations to demand fresh elections.
This article was published by BBC News.