"If the authorities don't respect the will of the people and fulfil their international obligations, then we will demand the resignation of the government," Sardar Jalaloglu, leader of the Azerbaijan Democratic Party, shouted from an orange-draped tribune on Wednesday.
The crowd of up to 15,000 people roared in approval and waved orange flags copied from the Orange Revolution in Ukraine last year in which huge crowds peacefully toppled an entrenched, corrupt government.
Opposition leaders called on US President George Bush and European powers to back their fight for the annulment of Sunday's parliamentary elections, which were described by Western observers as fraudulent.
"Today Azerbaijan is not alone. The West is with us," said Isa Gambar, head of the Musavat Party. "I call on Bush. You always say you are for free elections and democracy."
The opposition had predicted a crowd of 50,000; but despite the far smaller turnout, the demonstration on Victory Square showed plenty of spirit.
The republic sandwiched between Russia and Iran has been ruled by President Ilham Aliyev or his father Heydar Aliyev for the last 12 years, and Wednesday was the first time the usually squabbling opposition groups managed to unite in one rally.
Ilham Aliyev's party retained
power in Sunday's poll
"We want this regime to go," said Faik, a doctor who would not give his last name. "They have the force, the army and the money, but we have the voices."
"Resign!" and "Freedom!", the crowd chanted repeatedly.
Despite fears of violence, the approximately 500 riot police stationed in Freedom Square did not intervene.
Western monitoring groups described Sunday's elections, in which Aliyev's own party retained its dominant position, as deeply flawed, with reports of ballot stuffing, intimidation and multiple voting.
This gave moral support to the opposition and put the government, which is trying to deepen commercial and military links with the West, on the defensive.
Opposition's Ali Keremli (R) and
Isa Gambar rejected the results
However, it remained unclear whether the opposition had the stamina and resources to pursue an Orange Revolution strategy of non-stop protests.
Part of the crowd on Wednesday wanted to set up camp - something that would almost certainly have provoked violent dispersal by the police - but the opposition leaders called on their supporters to leave.
The independent Zerkalo newspaper wrote: "Much will depend on the behaviour of the opposition. If the opposition finds the strength to go to the end and achieve some real results, then probably this will fundamentally change the country."
However, should the opposition revert to previous form over recent years and give up, then "many of its representatives should probably prepare themselves for the political rubbish heap".
Although the opposition is calling for another demonstration on Saturday, some analysts believe that the political standoff will end in horse-trading rather than revolution.
"The opposition are using political bluff to try to get more seats in parliament," independent analyst Zardusht Alizade told AFP.
About 15,000 people joined the
protest in Baku's Victory Square
"A crowd of 15,000 is not a bad figure to start with, and the demonstrations could get bigger. However, the opposition just wants seats and the government is worried about Western pressure, so I think it will end with a compromise."
Already, election officials have said they will cancel the results in two of Azerbaijan's 125 constituencies and do a recount in a third.
Two heads of local administrations were sacked on Wednesday and four other officials were arrested amid probes into the election fraud.
Analysts say that as many as 20 to 30 constituencies may see re-votes, opening the way for opposition parties to improve on the mere 10 seats won on Sunday, according to official results.