The Prime Minister said on the other hand, the opposition could accept the results that benefited them despite claiming the polls were tainted by fraud.
"If the electoral system was tainted, hit by fraud, why have the swearing-in in Penang and Kelantan, and engage in a tussle (as to who should be sworn-in as menteri besar) in Selangor? Why accept (the results) in Permatang Pauh, Gelang Patah?
"The same system is used at the state and parliamentary levels. If the results benefited them, (they) accepted the results. If they were in BN's favour, they objected them.
"This is the greatest hypocrisy," he said at Umno's 67th anniversary celebration at the Putra World Trade Centre here.
Najib said, as an advocator of democracy, the opposition should accept the people's decision based on the system adopted under the Constitution.
Najib, who is also BN chairman, said before GE13, he had stated that he was ready to accept the decision of the people.
"They (the opposition) talked a lot about democracy, they had said that the people would determine everything... after the election, they refused to accept the results of the parliamentary democracy decided by the people," he said.
He also hit out at the opposition for giving various excuses for the defeat, like alleging that the BN brought in Bangladeshis as voters, and questioning the use of the indelible ink which they were the ones who had demanded for its use, in the first place.
"They resorted to lies on social media, spread stories by word of mouth to the extent that even intellectuals were taken in. This is the bad way of the opposition," he said.
The Prime Minister said, instead of spreading lies on social media, they (the opposition) should have taken their claims to the court.
Najib also chided the opposition leader for putting the people in difficult situations and tarnishing the country's image after failing to take over the reins of the government, since the last four general elections.
"So, if you want to do lecturing abroad, go ahead. Why wait any longer.
You have failed four times, why continue to hold your position?
"Is this what is called practising democracy? Is this the fight for a fair and open system?
That there is no corruption and nepotism (in your party)?
"But today, the (PKR) deputy president claimed that there is nepotism in the party.
This is double standard," he said. - Bernama