Former Colombian President Alavaro Uribe has described the failed peace deal with leftist rebels as "weak" following a meeting with President Juan Manuel Santos.
Mr Uribe, who led the campaign against the accord, said the deal had to be for everyone not half the population.
Tens of thousands of demonstrators have held marches urging the government and Farc not to go back to war.
Colombians narrowly rejected the deal in a referendum on Sunday.
President Santos is scrambling to salvage the deal, which opponents said was too soft on the Farc guerrillas.
Government negotiators have already returned to Havana for further talks with Farc leaders.
However Mr Santos said peace was "close" and that he would continue meeting opponents to try to salvage the deal.
His meeting with Mr Uribe was believed to be the first since he was sworn in as president six years ago. The one-time allies became bitter rivals after President Santos decided to negotiate with the Farc.
Marches for peace
Mr Santos has been supported by demonstrators who rallied in 14 cities demanding that the deal be saved.
Twenty-six public and private universities took part in the marches, organised by students on social media.
Many carried candles and white flags and walked silently, while some carried pictures of loved ones who were among the 220,000 killed during more than 50 years of conflict.
Finding forgiveness - Wyre Davies, BBC News, Bogota
If anyone has the right to feel angry and not to forgive, it is Edgar Bermudez.
At the height of the conflict between the Colombian government and left-wing Farc guerrillas, Mr Bermudez was on patrol in a rural area in the south of the country when he stepped on a land mine.
But he is worried about what kind of country his girls will grow up in now the peace deal is hanging in the balance.
For their part, the Farc have said that "peace is here to stay".
On Tuesday, President Santos announced that he would extend a bilateral ceasefire between the Farc and government forces until the end of October.
This prompted Farc leader Timochenko to tweet: "@JuanManSantos announces that the ceasefire with the @FARC_EPueblo will last until 31 October, and from then onwards does the war continue?"
But Colombia's ministry of defence released a statement on Wednesday clarifying that the ceasefire had been extended "initially" until 31 October and that it could well be extended beyond that date.
What does the 'no' campaign want?
Among the "corrections" those opposed to the deal have demanded are:
- That those found guilty of crimes be barred from running for public office
- That Farc leaders serve time in prison for crimes committed
- That the Farc use their illicit gains to pay their victims compensation
- That no changes be made to the Colombian constitution
The peace agreement was reached after four years of formal talks in the Cuban capital, Havana, between government and Farc negotiators.
From early on in the negotiations, President Santos announced he would put the final agreement to the Colombian people in a "yes" or "no" vote.
Polls suggested the agreement would be approved by a comfortable margi, but the deal was rejected by 50.2% of voters.