Kuala Lumpur - Pressure mounted on Sunday on Malaysian Prime Minister Najib Razak as influential former leader Mahathir Mohamad threw his weight behind mass protests demanding the premier's ousting over a financial scandal.Tens of thousands of Malaysians clad in the yellow of the pro-reform movement have turned out for two days of weekend demonstrations to demand Najib's removal, shutting down the city centre in a carnival atmosphere of speeches, sing-a-longs and prayer.But the 90-year-old Mahathir stole the show on Sunday, arriving to a rapturous welcome from protesters.In a hastily arranged news conference, Mahathir apologised for aiding Najib's rise to power, accused him of abusing his position to avoid corruption charges, and backed the protests."[Najib] will not step down. He knows when he has no power he may be made to face the court. The court may find him guilty and he may have to go to jail," Mahathir told reporters.Najib has been under fierce pressure since the Wall Street Journal last month published Malaysian documents showing nearly $700m had been deposited into his personal bank accounts beginning in 2013.Najib initially rejected the revelation, but his cabinet ministers have since admitted the transfers, calling them "political donations" from unidentified Middle Eastern sources.The accounts have been closed and the fate of the money remains unknown.Demanding transparency "I am here to demand transparency," said demonstrator Mustapha Abdul Jalil, 40, a businessman."This country is heading for bankruptcy and we must stop Najib and topple the corrupt regime." Najib had already faced months of allegations that huge sums of state money disappeared from deals involving a government-owned investment company he launched in 2009.Investigations into the various allegations appear stalled, however, after Najib in late July sacked or reassigned officials and parliament members who had launched probes.Najib denies wrongdoing and says he is the target of a "political conspiracy."Najib also has been bruised by the ringgit currency's fall to 17-year lows due to concerns over future Malaysian economic growth and the stigma of the funding allegations.But he struck a defiant tone on Sunday night in an annual address for Malaysian National Day, which is Monday, rejecting street protests as a means of expression and defending his economic record."We will never allow anyone, whether inside or outside the country, to ruin all that we have built so far," he said.Mahathir still casts a long shadow after helming the ruling coalition from 1981-2003.His tenure saw rapid economic development but - according to his many critics - crony capitalism became entrenched and the independence of key institutions like the judiciary was subverted.He has fiercely criticised Najib for more than a year, accusing him of corruption, abuse of power and economic mismanagement.Mahathir's support for the demonstrators raises the pressure on Najib, who took power in 2009 only after Mahathir engineered the ouster of his own chosen successor.But Najib's position is considered secure for now.He recently purged critics in his cabinet and has firm control of the powerful ruling party, the United Malays National Organisation (UMNO).The reform movement behind the protests, meanwhile, lacks much traction in UMNO's rural strongholds, and the opposition is currently fractured.'Black coal on Malaysia's face'Najib on Saturday denounced the demonstrators as people who "scribble black coal on Malaysia's face to the outside world”. The demonstrations that began on Saturday in the capital, and smaller gatherings in other cities, have been mostly incident-free despite police branding them illegal and banning the movement's yellow T-shirts.But Deputy Prime Minister Zahid Hamidi, who is also home minister in charge of domestic security, warned organisers may face charges.The organisers - electoral-reform activist group Bersih (the Malay word for "clean") - said 200 000 turned out in Kuala Lumpur at the peak on Saturday. Police put the number at 29 000.Demonstrators also denounced Najib over an unpopular new consumption tax, and demanded reform of an electoral system said to favour the UMNO-led ruling coalition.The coalition has controlled multiracial Malaysia since independence in 1957.But it has rapidly lost voters in recent years, particularly minority Chinese, over corruption, civil liberties curbs, and controversial policies favouring the ethnic Malay majority.Najib had vowed to end corruption and authoritarian tactics and to reform the pro-Malay policies.Following a 2013 election setback, however, he abandoned those initiatives under pressure from UMNO conservatives, including Mahathir.Mahathir, whose ultimate motives remain a subject of debate, took an uncompromising stance towards civil disobedience as leader but said the protests against Najib, were necessary because he was flouting the law.