Elections loom

Petro's dismissal was declared in December by right-wing prosecutor Alejandro Ordonez over the mayor's ill-fated attempt to reclaim the management of waste collection from private operators. Poor planning resulted in garbage amassing in the streets before the duties were returned to private contractors.

But trash quickly became a side issue as Colombians questioned whether the harsh sentence, especially the long ban from public office, was really intended to sideline the political left and bar a potential 2018 presidential candidate.

"(Santos' decision) shows that he is unable to bring peace to Colombia," Petro said in a speech from the balcony of the city hall, adding that the political establishment was unable to accept "diversity". He branded the decision "a political, electoral calculation".

Santos initiated peace talks with the main guerrilla group, the Revolutionary Armed Forces of Colombia, in November 2012, with the rebels' political participation one of the five points on the agenda on which partial agreement has been reached.

Center-right Santos will seek a second four-year term at elections on May 25, during which he hopes to complete the peace process aimed at ending a five-decade conflict that has killed around 220,000.

Surveys show Santos is the clear front-runner in the upcoming election and far ahead of Oscar Ivan Zuluaga, the chosen candidate of ex-president Alvaro Uribe. Uribe staunchly opposes peace negotiations and advocates continuing the policy of his 2002-10 term of defeating the guerrillas militarily.

Whether Santos dismissed or tried to maintain Petro in office, he was certain to draw the ire of politicians further to the left or right and the case could potentially cost him votes among some left-leaning voters.