Acting on an anonymous tip, police arrested Rana Abdel Rahim Koleilat, 39, on Sunday at the Parthenon Accor Hotel in Brazil's Sao Paulo city, police inspector Nicanor Nogueira Branco said in an official statement.
Koleilat, who was carrying a false passport identifying her as Rana Klailat of Northern Ireland, offered police up to 200,000 Brazilian reals ($94,000) to release her and was arrested for attempted bribery, the police statement said.
The Lebanese consul general in Sao Paulo, Joseph Sayah, said Koleilat was wanted in Lebanon for bank fraud and for questioning by the Independent International Investigation Commission in relation to the truck bombing that killed al-Hariri and 20 other people in central Beirut in February 2005, according to police.
"Her questioning is vital to clarify" al-Hariri's assassination, Branco said in the statement.
The commission was created by the UN Security Council in April 2005, soon after al-Hariri's killing.
Brazilian authorities were consulting British officials to determine whether Koleilat's passport was legitimate, Branco said. The passport said it had been issued in the Lebanese capital of Beirut by the British Embassy there in 2002, and listed Koleilat as a "British Overseas Citizen".
British officials in Brazil did not immediately respond to a message seeking comment on the validity of the passport.
The passport showed she had travelled to China, Egypt, Iraq and Turkey, Branco said. She visited Brazil three times over the last year or so, but "no one had any idea she was here in Sao Paulo. It's a huge city and it's easy to disappear here", he said.
Sao Paulo has 18 million residents, and is home to a large community of Brazilians of Lebanese descent.
"...no one had any idea she was here in Sao Paulo. It's a huge city and it's easy to disappear here"
Nicanor Nogueira Branco,
Police inspector, Sao Paulo
Interpol asked Brazilian police on 3 December to try find Koleilat, but no arrest warrant was issued, Branco said. The passport showed she most recently entered Brazil on 10 December.
Branco said Koleilat spoke fluent French, English and Arabic but very poor Portuguese, the Brazilian language. She was being held in a police station that does not have jail cells. Authorities did not let her speak with reporters, though she was visited on Sunday by a lawyer Branco did not identify.
He said police seized about 15,000 Brazilian reals ($7000) she was carrying and took her mobile telephone to try trace her calls.
Branco said he did not know if Koleilat would be extradited or prosecuted first in Brazil on the bribery accusation. She was arrested for attempted corruption, and faces up to eight years in prison if convicted.