Mr Patarkatsishvili backed Georgia's 2003 "Rose Revolution"

Valery Gelbakhiani runs the campaign of tycoon and presidential hopeful Badri Patarkatsishvili, who is an arch-rival of President Mikhail Saakashvili.

Mr Gelbakhiani is "a suspect in organising a coup d'etat," deputy chief prosecutor Nika Gvaramia said.

Mr Saakashvili set the election for 5 January after huge opposition protests.

The authorities had earlier accused Mr Patarkatsishvili, who now lives in Britain, of plotting a coup to overthrow Mr Saakashvili, though he was not charged.

Mr Saakashvili says he wants elections to provide a fresh start

Mr Saakashvili sent in riot police to quell the street protests in November. He also imposed a state of emergency - later lifted - and alleged there was a hidden Russian hand in the unrest.

He then promised to bring forward presidential elections to 5 January 2008.

The authorities temporarily shut down Imedi TV, the main outlet for opposition views, which is co-owned by Mr Patarkatsishvili.

This week The Sunday Times reported details of an alleged plot to kill Mr Patarkatsishvili.

Referring to the alleged plot, Mr Patarkatsishvili said: "I appeal to the authorities of Georgia to start an immediate investigation into this matter and hold those behind the plot accountable for their actions".

President Saakashvili, a US-educated lawyer, came to power after street protests in 2003, nicknamed the Rose Revolution.

His first term as president has seen Georgia strengthen its ties with Nato while relations with Moscow have soured.

Opposition groups accuse him of authoritarian tendencies and a failure to tackle large-scale social deprivation in Georgia.