"For Mr. Jammeh, the end is here and under no circumstances can he continue to be president".
The spokesman for the United Nations chief in a statement Wednesday said Tuesday's blockading of the election offices was an "outrageous act of disrespect" and a sign of defiance as West African leaders visited to urge President Yahya Jammeh to accept his loss.
Incumbent and outgoing Yahya Jammeh was beaten by coalition candidate Adama Barrow ending Jammeh's 22 years as president.
Jammeh lost elections on December 1 and, despite initially accepting the outcome, has since changed his mind and is challenging the outcome at Gambia's Supreme Court.
"This action violates the independent status of the IEC under the Gambian constitution, and could compromise the sensitive electoral material under the commission's custody", he added.
Mohammed Ibn Chambas, the UN Special Representative for West Africa and the Sahel, told newsmen in Dakar that Jammeh would face strong sanctions if he tried to cling to power.
But in a dramatic about-face that drew worldwide condemnation, he then rejected the voting results last Friday, and his party is now challenging the outcome at Gambia's Supreme Court. Its front gate and ground floor entrances were closed.
The chairman of the IEC, Alieu Momar Njie, told AFP on Wednesday morning that he had not yet been informed why he was locked out of his own premises.
The election should be annulled, the ruling Alliance for Patriotic Reorientation and Construction (APRC) said in a document handed to the registrar of the Supreme Court.
The U.S. Embassy in the capital, Banjul, also demanded that security forces withdraw, saying the "unnecessary and unprovoked show of force is seen as a move to subvert the democratic process in the Gambia".
However, in a notable exception, United Nations troops intervened militarily alongside France to oust Ivory Coast's then-president Laurent Gbagbo after he used the constitutional court to overturn the 2010 election victory of Alassane Ouattara.
Al Jazeera's Nicholas Haque, reporting from Senegal, which borders Gambia, said that the visiting West African heads of state have left Gambia "empty-handed", without agreements or deals, despite serious efforts to pursuade Jammeh to provide a smooth transition on power.
Asked whether military intervention was an option in Gambia if mediation failed, Mr Chambas said: "It may not be necessary. Let's cross that bridge when we get there", he said.