Some 100 members of the elite Berkut anti-riot police unit were in position on the edge of the tented camp set up by Tymoshenko supporters in central Kiev as rumours spread that the authorities could forcibly remove the protest.
In a tense stand-off, around 100 supporters of the ruling Regions Party also arrived on the scene to stage a counter action, an AFP correspondent said.
"We are going to stay despite the attempts by the authorities to remove us. We are not scared, we just want our children to be able to live in a normal country," said Tymoshenko supporter Galina Zanko, 58.
The protest on the capital's central Kreshchatyk avenue had been ruled illegal overnight by the Kiev district administrative court. However despite the tension there was no sign of police moving in to disperse the action.
A Kiev court on Friday ordered the arrest of the former prime minister over a month into a her trial on charges of abuse in power which she claims is part of a political vendetta pursued by by her rival, President Viktor Yanukovych.
Yanukovych became president last year after defeating Tymoshenko and other leaders of the 2004 pro-Western Orange Revolution in bitterly-contested elections.
Since coming to power, Yanukovych has sought to dispel Western fears over his pro-Kremlin leanings by setting a clear aim of EU integration. But the arrest has raised alarm among Ukraine's partners.
Echoing concern from the European Union, the United States said her arrest "raised concerns internationally about the application of the rule of law in Ukraine and further contributes to the appearance of politically-motivated prosecutions."
Tymoshenko also found heavyweight support from Ukrainian boxing star Vitali Klitschko who announced he halted training for an upcoming fight to take part in a "battle for democratic values" following her arrest.
The trial of Tymoshenko, who spent a second night in the Lukyanovsky detention centre in Kiev, is due to resume at 0700 GMT on Monday in what promises to be a highly-charged hearing.
Copyright AFP (Agence France-Presse), 2011