* Biggest rally in years after small protests broken up
* Chants echo Arab Spring demands from across region
* Police present but did not stop the rally
By Khalid Abdelaziz
KHARTOUM, June 29 (Reuters) - Thousands of Sudanese called for the overthrow of veteran President Omar Hassan al-Bashir on Saturday, spurred on by an opposition trying to stoke an Arab Spring style uprising.
The opposition, capitalising on anger over soaring food prices and corruption, has threatened to stage mass protests to topple Bashir within 100 days.
The uprisings that shook the Arab world have passed Sudan by as the security forces usually break up the frequent small street protests by students before they have a chance to spread.
But on Saturday, several thousand people - possibly as many as 10,000, according to witnesses - rallied in a square in Khartoum's twin city Omdurman, the biggest rally in years.
Echoing the language heard across Tunisia, Egypt and Libya during mass demonstrations that overthrew leaders there, protesters held up signs saying: "The people demand the fall of the regime" and "Go Bashir".
"We tell this regime - you have to go," Sadiq al-Mahdi, head of the opposition Umma Party, told the crowd, shouting back "Go, go" as he was speaking.
"This regime has failed on all levels during a quarter of a century in power," he said. Sudan's last democratically elected prime minister was toppled by Bashir in a coup in 1989.
A Reuters reporter saw hundreds of police officers at the scene but they did not stop the rally.
Bashir still enjoys the support of the army and influential Islamist groups. He dismisses the opposition parties as insignificant.
Critics of the opposition say its leadership fails to pose a challenge because it is more absorbed by its own rivalries than organising sufficiently to form a potent challenge to the elite.
Several leaders such as Mahdi have past government ties, sapping the opposition's credibility among young people.
Sudan's economy has been in turmoil since the former civil war foe South Sudan seceded in 2011, taking away most oil production, which used to be the main source for the budget and dollars needed to fund food imports. (Writing by Ulf Laessing; Editing by Alison Williams)