Tens of thousands of protesters pressed ahead with a mass rally in Bangkok on Tuesday, some expressing frustration that four days of peaceful protests had failed to force Thailand's premier to call elections.

After marching on Monday to an army base that Prime Minister Abhisit Vejjajiva has made his headquarters for the crisis, red-shirted supporters of ousted ex-premier Thaksin Shinawatra regrouped near a central Bangkok bridge where they set up camp.

Financial markets have so far held fairly steady and foreigners continued to buy Thai stocks on Monday, albeit in smaller volume than over the past 15 sessions, when they have bought a net $850 million (564.6 million pounds) despite the rise in political tension.

Uncertainty over the intentions of the "red shirts" caused volume to drop, although not sharply.

"It looks like the situation overall is manageable. If we can get past this week, things should return to normal and fund flows should start building up again," Patareeya Benjapolchai, president of the Stock Exchange of Thailand, told Reuters.

"Although we've seen a slower pace of foreign buying because of politics, this will not be enough to trigger a major sell-off -- unless of course the situation gets out of hand and people start getting shot," she added"

"Fundamentally, Thailand isn't bad looking."

That message was reinforced by U.S. investment bank Morgan Stanley which upgraded its recommendation on Thailand's stock market to "overweight" in a note to clients dated March 1 2 and distributed to media this week.

Early on Tuesday, leaders of the "red shirts" were collecting a vial of blood from each supporter which they intend to spill outside Abhisit's office in what they are calling a symbolic shedding of blood for democracy.

"When the picture of this reaches Abhisit, won't he be just a little bit ashamed of himself?" said Rung Suramanee, 76, a retired civil servant. "I am ready to sacrifice anything for majority rule to return. What's 10 cc of blood?"

The mainly rural "red shirts" say they have been disenfranchised by the military, urban elite and royalists, who wear yellow shirts at protests and broadly back Abhisit.

some businesses and sap consumer confidence in Southeast Asia's second-biggest economy, possibly forcing the central bank to delay an expected rise in interest rates. Such a delay, however, is looking increasingly less likely given the peaceful protests.

Some protesters expressed frustration about the seeming lack of effect of the rally, even though 150,000 people massed in Bangkok at one point on Sunday evening. Some said they may soon return home because they cannot leave work and family for long.

"I want to step it up and do something bold," said Manat Tengmanee from northern Lampang province. "I think without real splattering of blood there will be no change. There has never been change for the people in Thailand without bloodshed." But he insisted he would follow the leaders' calls for non-violence.

The twice-elected Thaksin was ousted in a military coup in 2006 and later sentenced in absentia to two years jail for graft. He fled into exile shortly before his sentence was passed and lives mainly in Dubai, although thought now to be in Europe.

In a 40-minute address by telephone on Monday night, during which he broke into song about his love for his supporters, the self-exiled former telecommunications tycoon urged the "red shirts" to keep up their struggle.

"The patience of the people is the heart of success," he said. "Do not lose heart yet. Be patient. It may be a little tough," he said, before urging Abhisit's coalition partners to withdraw from the government "for the sake of democracy."

Thaksin's allies are likely to win the next election, which must called by the end of next year, just as they have won every poll held since 2001. Thaksin's opponents could thwart that result, possibly with a coup, as in 2006, or a judicial intervention, as in 2008, a year plagued by political upheaval.

(Additional reporting by Chalathip Thirasoontrakul and Ploy Ten Kate; Writing by Alan Raybould; Editing by David Fox)