By Yaroslav Lukov
BBC News website, Kiev
The orange army was ecstatic watching the big TV screen set up on Kiev's main Independence Square when opposition leader Viktor Yushchenko finally appeared at his headquarters, well after midnight.
"It has happened! This is the victory of the Ukrainian people!" Mr Yushchenko said to loud cheers from reporters.
Mr Yushchenko urged the crowds to stay to defend victory
He later went on to the Independence Square - known here simply as Maidan - to thank his supporters personally.
But he urged Maidan's tent city to stay on to be ready to defend the victory.
The crowd had already been chanting "Yushchenko - President!" for some time, realising that the gap between their candidate and Prime Minister Viktor Yanukovych - although based on exit polls only - could hardly be breached.
In fact, the party started soon after the poll ended at 2000 (1800 GMT) with thousands of Mr Yushchenko's supporters pouring to the square in response to his earlier call.
People were singing and dancing as some of Ukraine's best-known musicians performed on stage in anticipation of Mr Yushchenko's victory.
And they exploded with loud cheers when the results of three exit polls were announced.
"We're winning, we're winning!" shouted a group of young girls standing next to me, flashing V-signs for victory.
"We can scent our victory in the air! Soon we'll be able to fully enjoy the taste of it!" said one of the musicians.
Other people were more cautious, predicting a delay in announcing of the official results because of possible legal wrangling.
"We'll stay on Maidan until Mr Yushchenko is sworn in as next president," said Taras, who has been living in the tent city for several weeks.
As polls closed, a grim-looking Mr Yanukovych gave a news conference at his Kiev headquarters.
"I'm waiting for my victory, but if I lose I am absolutely sure... that any negotiations [with Mr Yushchenko's camp] are senseless," he said.
The prime minister - who had earlier repeatedly urged his rival to make a deal - said he was ready to form an opposition and defend relentlessly his voters' rights.