A day after taking office, President Enrique Pena Nieto signed a pact with rivals Sunday to strengthen democracy, in what could be a bold break with his party's bad old days.

The "Pact for Mexico" was agreed after hard bargaining among Pena Nieto's Institutional Revolutionary Party (PRI), the conservative National Action Party (PAN) and the leftist Democratic Revolution Party (PRD).

The deal, which Pena Nieto and the opposition leaders signed in Mexico City's Chapultepec Castle, has a five-point plan including fundamental rights; justice and security; anti-graft measures; and economic growth and competitiveness.

Pena Nieto's presidency marks the return of the PRI, a once autocratic party that ruled non-stop from 1929 to 2000 with a mix of patronage, corruption and rigged-elections. The new leader insists that the PRI has embraced democracy.

So "this pact gives Mexico stability, certainty and a path," said Pena Nieto, who was elected to a single six-year term on July 1.

The agreement divided the PRD, whose candidate in the election, Andres Manuel Lopez Obrador, finished second but refused to concede defeat. The top electoral tribunal threw out his bid to overturn the result over vote-buying claims.

Pena Nieto's inauguration on Saturday was met with some violent protests.

In his inaugural speech, Pena Nieto unveiled a 13-point plan to transform Mexico, centered on a strategy to reduce runaway drug violence, boost the economy and reduce poverty.

Since 1997 legislative elections, none of Mexico's political parties has managed to secure an absolute majority in either chamber of Congress, putting a break on many reforms.

The opposition leaders warned that Pena Nieto would not get an easy ride.

"It does not put conditions on our role of critic of the new government," said PAN leader Gustavo Madero, whose party held the presidency for the last 12 years but finished third in the July election.

The presidency of Felipe Calderon was marked by economic growth but it was overshadowed by a drug war that has left a staggering more than 60,000 people dead since 2006.