Nearly 30 million Colombians were eligble to vote in the elections
Parties allied to Colombian President Alvaro Uribe are on course to remain the biggest parties in Congress after Sunday's legislative elections.
But a delay in declaring final results amid technical glitches has led to recriminations between candidates, the government and electoral officials.
The elections were seen as an indication of voter intentions for May's presidential election.
Mr Uribe is barred from standing for a third term.
The delay in declaring results provoked protests from candidates, especially from the opposition parties.
The government also weighed in, with Interior Minister Fabio Valencia saying the electoral authorities must fix the problems to guarantee the legitimacy of the presidential election on 30 May.
However, Alejandra Barrios, head of an election monitoring group, MOE, said it was irresponsible to cast doubt on the electoral authorities.
She told BBC Mundo the government should carry out electoral reform which "has been needed for years".
Sunday also saw the Conservative Party and the Green Party hold primaries to choose their presidential candidate.
Noemi Sanin and Andres Felipe Arias, who are battling to be the Conservatives' choice, both accused the electoral authorities of "ineptitude" after counting in their race was halted and final results not expected until the end of the week.
Election day itself was marred by accusations of vote-buying. Observers from the Organisation of American States (OAS) said they had seen such practices in six of the 16 provinces they monitored.
The parties that form Mr Uribe's governing coalition, the U Party and the Conservatives, secured the most seats in the Senate and the Chamber of Deputies, and any recount would not change the overall result.
The result is a boost for Juan Manuel Santos, a former defence minister who has pledged to continue Mr Uribe's tough security policy if he wins in May.
However, opinion polls suggest he is unlikely to win outright in the first round.
Meanwhile, a new party accused of links to far-right militias has emerged as a political force, winning eight Senate seats.
The Party of National Integration (Pin) is composed of family and friends of former members of Congress who have been jailed or are under investigation for alleged paramilitary ties.
"Under no circumstance does the party have anything to do with the guerrillas, or with paramilitarism, or drug-trafficking," Pin's legal representative, Alvaro Caicedo, told the Associated Press news agency.