She said the rally had an "atmosphere more of a muddy festival than an angry protest".
Hundreds of police have been deployed to the area and the army has been put on stand-by in case of clashes between the protesters and government supporters, although rally organisers have said there will be no violence.
|The Red Shirts have held protests against the government of Abhisit Vejjajiva [EPA]
Red Shirt leaders plan to submit a petition to the Thai king later in the day which they claim has five million signatures calling for a royal pardon for Thaksin.
The move has been criticised by opponents who say it is an attempt to drag Thailand's widely-revered royals into a political argument.
Whether or not the pardon is granted, protest organisers said Monday's rally would remain a visible demonstration of the support Thaksin still carries in Thailand and opposition to the current government.
At around midday on Monday Thaksin, who was deposed in a bloodless coup in 2006, addressed the crowd of supporters by telephone, although our correspondent said it was unclear where he was speaking from.
Thaksin returned to Thailand in early 2008 but fled the country months later, shortly before Thailand's supreme court found of him guilty of conflict of interest and sentenced him to two years in jail.
He has been in exile ever since travelling between residences in, among others, Dubai, Hong Kong and Nicaragua - which has appointed him a special ambassador to promote investment in the central American country.
The pro-Thaksin Red-Shirts and their rival Yellow-Shirts who support the current government have conducted a series of demonstrations in the Thai capital, effectively bringing the city to a standstill.
Thaksin's supporters say that the government of Abhisit Vejjajiva, the current prime minister, is illegitimate and the result of a parliamentary stitch-up engineered by the army.
They remain angry over the way Abhisit took power after a court ruling that removed a government made up of Thaksin allies from power.
That anger culminated in deadly street battles in April that pitted the Red Shirts against the Thai army.
At least two people were killed and more than 100 injured in what was the worst violence seen in the country for decades.
Four prime ministers in the last 15 months have failed to resolve Thailand's deep political rift which continues to pit the military and business elite against a rural majority loyal to Thaksin.