* Retail investors can sign up online or at local banks
* Retail will be sold up to 0.5% of state oil giant
* Marketing blitz includes endorsement by clerics (Adds colour and investor comment)
RIYADH, Nov 17 (Reuters) - "No voice is louder than that of the Aramco IPO, the largest IPO on earth," declared Ahmed al-Arfaj, a Saudi Arabian TV talk show host, drumming up demand for what could be the world's biggest initial public offering.
State oil giant Saudi Aramco began its repeatedly delayed share sale on Sunday, offering retail investors and institutions a stake in the world's most profitable company through a listing on Riyadh's Tadawul bourse.
Saudis clamouring to own part of the world's top oil exporter can go online or to local banks, which have extended working hours to meet high expected demand during a sale process that runs until Nov. 28.
Retail investors will be sold up to 0.5% of Aramco, about $8.5 billion worth of shares, in the IPO, which values the company at $1.6 trillion to $1.7 trillion.
Ahmed Sanad, 37, who signed up himself and his family on Sunday at a Saudi British Bank (SABB) branch in central Riyadh called the IPO "the talk of the town" and "a global event".
"I dont think anyone will miss the opportunity," he told Reuters. "There are no better options at the current time."
Roadside billboards and shopping mall adverts have trailed the listing for weeks, hailing "Saudi Aramco, soon on Tadawul". On social media, in cafes and at family gatherings across the country, it has dominated conversations.
The marketing blitz contrasts with the other bumper share sale this month, the secondary listing of Chinese retail titan Alibaba, which is not expected to carry out an advertising campaign for what will be Hong Kong's first paperless listing.
Abdullah al-Faqeeh, a 29-year-old dentist, said he began saving up to buy shares once de facto Saudi ruler Crown Prince Mohammed bin Salman announced the plans nearly four years ago.
"I will invest in the company for the long term," he told Reuters. "I will get bonus shares, and with the profits I will buy more shares. It is a once-in-a-lifetime opportunity."
Retail investors will be entitled to one bonus Aramco share for every 10 they buy if they hold on to their stock for at least 180 days. Up to 5 million people are expected to participate, local media reports.
Thousands of Saudis were seeking to invest on behalf of their dependents, to increase the number of shares they can buy, two bankers told Reuters.
Saudi banks are also marketing loans, with some offering four times the usual limits, two other financial sources said, adding that they are able to do so as they will hold the stock on behalf of clients so have it as security.
Over the weekend, senior Muslim clerics including royal court adviser Sheikh Abdullah al-Mutlaq declared that investing in Aramco was permissible.
"It (Aramco) is a pillar of the Saudi economy," Mutlaq, a member of the kingdom's top clerical body, said on a weekly Friday radio show in response to a caller's question. "I think even the scholars, we will participate in it."
RIYADH OIL RUSH
Aramco said on Sunday it plans to sell 1.5% of its shares or about 3 billion shares, at an indicative price range of 30 to 32 riyals, valuing the IPO at as much as 96 billion riyals ($25.6 billion) at the top end of the range. It could beat Alibaba's record $25 billion IPO in 2014.
The government has been pushing rich Saudis to invest with cash held abroad, with many viewing it as an opportunity to show their patriotism after a September attack on Aramco facilities that struck at the heart of the kingdom's energy industry.
Washington and Riyadh blamed regional rival Iran for the attacks, which temporarily cut more than 5% of global oil supply. Tehran has denied any involvement.
"Participating in the Aramco IPO is a national duty for whoever can afford it," Saudi columnist Anwar Aboalela tweeted.
Another Twitter user, Meshal Althaidy, described Prince Mohammed's push to take Aramco public - the centrepiece of a reform agenda aimed at weaning the economy off oil - as "a fierce battle" that Saudis should support.
Although some locals think the government is wrong to relinquish even partial control of Aramco, the prospect of holding shares is hard to resist.
"Aramco will be a winning horse," said Uber driver Abu Mohsen. "I will subscribe only because of the expected gains but I remain against the sale, especially to foreigners."
(Reporting by Marwa Rashad; Additional reporting by Stephen Kalin in Riyadh and Saeed Azhar in Dubai; Editing by Pravin Char and Alexander Smith)