Mahathir has thrown in his lot with an opposition alliance packed with parties he crushed while in power, electrifying an election race that had long looked like an easy win for the ruling Barisan Nasional coalition.
All 222 parliamentary seats and 505 state seats across 13 state assemblies will be up for grabs on Wednesday.
Najib has been cleared of any offence by the attorney general.
Najib's speech was broadcast live on television while Mahathir's was delivered over Facebook.
Almost 15 million Malaysians will go to polls today to elect 222 members of the 14th federal parliament and representatives for 12 state legislatures.
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The results are expected to come out later on Wednesday, at the earliest.
Al Jazeera's Florence Looi, reporting from the capital, Kuala Lumpur, said it was the first time voting was held mid-week. Indeed, Malaysian politics revolves around UMNO, which Najib leads.
But Najib tweeted that many BN leaders' phones had been hit by similar attacks while the internet regulator blamed anonymous automated "bots" and said it was investigating.
And because RoS also rejected an application by Pakatan to be registered as a coalition, the bloc's four member parties later agreed to contest the election together under the symbol of the People's Justice Party (PKR), which is headed by jailed leader Anwar Ibrahim. "Our campaign workers are also affected".
Today leading independent pollster Merdeka Center released an opinion survey indicating that the ruling coalition would maintain power but lose the popular vote, for the second consecutive general election.
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Prime Minister Datuk Seri Najib Razak is confident that Barisan Nasional (BN) will get the mandate to lead the nation for the next five years.
Opponents said electoral boundary changes rushed through parliament last month favor the National Front by shifting likely opposition voters into seats where the opposition already prevails.
A previous opposition grouping fell apart in 2015 amid a dispute between the DAP and the Pan-Malaysian Islamic Party (PAS) over Muslim law. Last week Umno sacked two senior leaders and suspended another - all long-time loyalists of Mahathir - for speaking against Najib and attending opposition events.
He wrote that the 92-year-old former prime minister's speeches "contain little policy substance but maximum slander", while the DAP-led opposition have obscured the truth "with a tsunami of fitnah (lies)". PAS has gone its own way and is fielding candidates separately at the poll, which could split the opposition vote. "Should they win, they will never let Mahathir become prime minister - they know him too well", Tengku Sariffuddin said in a statement.
Many Malays will continue to back BN as they have benefited from a decades-old affirmative action programme that gives them perks, such as priority for government jobs. USA investigators say $4.5 billion was stolen from 1MDB, the investment fund, by associates of Najib between 2009 and 2014, including $700 million that landed in Najib's bank account. One is the series of unsettling events related to the quasi-sovereign fund company 1Malaysia Development Berhad (1MDB), which is being investigated by several global jurisdictions (including the United States and Switzerland) for alleged abuses and widely reported by the worldwide media.
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