The UN chief is calling for national unity and reconciliation, not revenge, in Libya following the death of Muammar Gadhafi.
Secretary-General Ban Ki-moon said combatants on both sides "must lay down their arms in peace."
Gadhafi, who ruled Libya with a dictatorial grip for 42 years, was killed Thursday when revolutionary forces overwhelmed his hometown of Sirte.
"Clearly this day marks an historic transition for Libya," Ban said Thursday. "Yet let us recognize immediately that this is only the end of the beginning. The road ahead for Libya and its people will be difficult and full of challenges."
Ban pledged UN support to Libya's transitional authorities as they build a new nation.
The capture and reported death of Libya's longtime tyrant was arguably the most dramatic event of the so-called Arab Spring, an eventful period of revolt that began in Tunisia towards the end of last year.
Gadhafi, who had ruled with an iron fist since 1969, was arrested in his hometown of Sirte, broadcaster Al Jazeera quoted an official from the National Transitional Council as saying. The fugitive leader was believed to have been captured while driving in a convoy of cars fleeing from Sirte.
NATO, which has been enforcing a no-fly zone and supporting anti-Gadhafi forces with airstrikes, was also believed to have played a key role, confirming that its jets had bombed a military "convoy" near Sirte in the morning.
However, NATO officials said it was impossible to say whether the airstrike contributed to the possible death of Libya's on-the-run leader. "Since we do not have any boots on the ground, we are unable to say anything about the identity of any person killed," a spokeswoman said.
French President Nicolas Sarkozy said on Thursday that Gadhafi's death turned a page for the Libyan people and signaled the start of a democratic process.
"The liberation of Sirte must signal... the start of a process agreed by the CNT to establish a democratic system in which all groups in the country have their place and where fundamental freedoms are guaranteed," Sarkozy said in a statement.
Sarkozy, who spearheaded military intervention in Libya, said now was the time for "reconciliation in unity and freedom".
British Prime Minister David Cameron said that the death of the former Libyan leader held out the promise of a better future for the people he ruled for four decades.
"People in Libya today have an even greater chance after this news of building themselves a strong and democratic future," he said in a statement in Downing Street.
German Chancellor Angela Merkel said Thursday that Libya was now free to make a fresh start and begin peaceful democratic reforms after Gadhafi's death.
"This brings to an end a bloody war that Gadhafi waged against his own people. The path is now finally clear for a fresh political start, in peace. Germany is relieved and very happy about this," Merkel said in a statement.
She said Libya should now carry out political reforms to "ensure the achievements of the Arab Spring cannot be undone."
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