Clinton will discuss the language of diplomacy

Secret Service agents and the Metropolitan Police have been helped by Dyfed-Powys Police to put strict measures into place after the former president decided he would like to stage a walkabout in the town to meet visitors.

Mr Clinton - who is reported to command at least $100,000 per speech - will talk about conflict resolution and the language of diplomacy when he appears at the BBC Wales World Lecture at 1945BST.

His speech will be followed by a question and answer session.

On Friday Mr Clinton, whose tour of Britain and Ireland began last week, received a warm welcome on his return to Oxford University to open an institute for American studies.

He was accompanied by his daughter, Chelsea, who is thought to be interested in studying at the university.

McCartney poetry

Organisers of Mr Clinton's speech in Hay say they could have sold the 1,300 tickets - costing �100 - several times over.

It has been estimated that his presence at the festival in the town which has a population of just 1,300 will bring an extra �1.5m into the area.

Van Morrison is making a return visit to Hay

Sir Paul McCartney, who has recently published a book of poetry, will also be at the festival which attracts crowds from across the world.

Authors booked to appear this year include Louis de Bernieres, Margaret Atwood, Nick Hornby, Germaine Greer and Frank McCourt.

Irish rock star Van Morrison is due to perform on 1 June.

As a result of nearby foot-and-mouth disease outbreaks, the festival has been moved from its usual farm site on the outskirts of Hay to a less sensitive site in the town centre.