Indian opposition leader Rahul Gandhi on Wednesday dismissed as "fake" exit polls predicting a clear election victory for Prime Minister Narendra Modi, a day before the scheduled release of results.

A slew of exit polls released after the world's largest election ended on Sunday projected that Modi and his allies would return to power with between 282 and 313 seats out of 543 in parliament.

"My dear Congress party workers. The next 24 hours are important. Stay alert and vigilant. Don't be afraid. You are fighting for the truth," Gandhi, head of the Congress party, said on Twitter.

"Don't get disappointed by the propaganda of fake exit polls. Keep faith in yourself and the Congress party. Your hard work won't go to waste. Jai Hind (Bow to the motherland)," he wrote.

Modi's Bharatiya Janata Party (BJP) welcomed the predictions and Indian stock markets rose strongly on Monday, but exit polls in India are notoriously unreliable.

In 2004 they predicted that Prime Minister Atal Vajpayee's BJP-led government would be re-elected but results showed the opposite, bringing a Congress-led alliance to power under Manmohan Singh.

In the last election in 2014, the BJP won 282 seats, the first time a party had won a majority on its own in 30 years. It then cobbled together an alliance with a commanding 334 seats.

Counting of the roughly 600 million votes cast was due to begin at 8:00 am (0230 GMT) on Thursday. If there is a clear trend this should be evident by around midday.

- Voting machine doubts -

Party volunteers on Wednesday joined armed police to guard strongrooms containing the four million electronic voting machines (EVMs) used in the election before counting begins.

On Tuesday more than 20 opposition parties had called on the election watchdog to ensure the machines were not manipulated after video clips emerged on social media purporting to show irregularities.

The Election Commission moved swiftly to refute the reports, saying voting machines were "absolutely safe in strongrooms", and those shown in video clips were reserves.

Amit Shah, president of the BJP, said Wednesday that the opposition was rattled by their likely defeat and were "tarnishing" India by raising questions about the electoral process.

Vijay Singh, an election agent for the Samajwadi Party in Lucknow -- capital of the key state of Uttar Pradesh -- was one of those keeping an eye on the strongroom, a common practice during elections, despite the baking heat.

"We sit there in shifts of eight hours. The administration has provided a tent where we camp round the clock," Singh told AFP.

"We are the foot soldiers of the party and are always ready to serve our party under any circumstances. And we are there around the strongroom to ensure there is no security breach."

Indian government issued an advisory to the regional governments to be vigilant when the votes are counted Thursday.

The home ministry "alerted" the state governments and police chiefs "regarding possibility of eruption of violence in different parts of the country in connection with the counting of votes tomorrow".

"(The ministry) has asked the states and union territories to maintain law and order, peace and public tranquility," the government statement said.

"This is in the wake of calls and statements made in various quarters for inciting violence and causing disruption on the day of counting of votes," it added.