Betancourt is a severe critic of FARC

Ingrid Betancourt ignored military advice not to travel to the area, which for three years had been under the control of rebels of the Revolutionary Armed Forces of Colombia (FARC).

A campaign spokesman quoted by Reuters news agency said Mrs Betancourt had been taken "by the 15th front of the FARC".

Kidnapping is a favoured tactic of the rebels, who are believed to be holding about 800 people hostage.

Mrs Betancourt has been a severe critic of the FARC, who were ordered out of the safe haven after President Andres Pastrana declared the Colombian peace process dead.

As Colombian troops retook the area, President Pastrana visited the main town of San Vicente del Caguan on Saturday, promising no let up in the war against the FARC.

Unheeded advice

Mrs Betancourt, who is running as an independent in the poll on 26 May, travelled towards San Vicente - about 250 km (170 miles) south of the capital, Bogota - to attend a human rights rally on Saturday.

She was travelling with a French magazine photographer, a Colombian cameraman and two campaign aides.

Click here for a map of the former FARC safe haven

"The candidate and her campaign director (Clara Rojas) were kidnapped on Saturday," her spokeswoman Diana Rodriguez told the AFP news agency.

Three other people taken hostage by the FARC guerrillas were released, the spokeswoman added.

Mrs Betancourt's husband, Juan Carlos Lecompte, said she had felt she needed to be with the people of San Vicente "during the good and the bad".

The government had warned her not to go.

Interior Minister Armando Estrada said security forces were searching for Mrs Betancourt and doing what they could to establish security in the war zone.

"It is good that politicians are doing what they can to draw support for their campaigns and their causes ... but it was not necessary to make that trip in those conditions," Mr Estrada said, asking other candidates to refrain from visiting the area for the moment.

Mrs Betancourt is trailing last in the race for the presidency, according to opinion polls, but the BBC's Peter Greste says the kidnapping illustrates just how critical the situation remains.

The Colombian Government may have retaken the main towns in the former safe haven but the FARC rebels remain undefeated in the jungle, our correspondent says.