Counting of votes for the seven-phase Lok Sabha elections began Thursday morning across the country.

The counting exercise is on in over 4,000 counting centres amid concerns raised by opposition parties over alleged tampering of EVMs. They have asked the Election Commission to ensure transparency in counting.

Election Commission officials said results are expected only by late evening.

Prime Minister Narendra Modi, several union ministers, BJP chief Amit Shah, Congress president Rahul Gandhi and UPA chairperson Sonia Gandhi are among key leaders who contested the polls.

Most of the exit polls have predicted that the BJP-led National Democratic Alliance (NDA) is on course to retain power for a second term.

However, the opposition parties have trashed results of the exit polls, claiming that the BJP will suffer severe drubbing due to rising unemployment, farm distress and slowdown in the economy.

The voting was staggered between April 11 and May 19 in which around 67 per cent of the nearly 900 million eligible people exercised their franchise to elect 542 members of the Lok Sabha from a total of 8,049 contestants.

In the 2014 elections, the BJP won 282 seats while the Congress had suffered a severe drubbing, getting an all-time low of 44 seats as against 206 it won in 2009.

For the first time in Lok Sabha polls, the EC will tally vote count on Electronic Voting Machines with voter verified paper audit trail slips in five polling stations in each assembly segment of a parliamentary constituency.

It will effectively mean that out of nearly 10.3 lakh polling stations, the EVM-VVPAT matching will take place in 20,600 such stations.

In case of a mismatch, the results based on paper slip count will be considered as final.

The entire exercise of EVM-paper trail machine matching will take an additional four to five hours, EC officials said.

They said the voting percentage of 67.11 this time was the highest ever-voter turnout in Indian parliamentary elections.

They said according to procedure, postal ballots would be the first to be counted.

The number of service voters stands at 18 lakh and these include personnel of the armed forces, central police force personnel and state police personnel who are posted outside their constituencies.

Diplomats and support staff posted in Indian embassies abroad are also counted as service voters.

Out of the 18 lakh registered voters, 16.49 lakh have sent their postal ballots to their respective returning officers as on May 17.

The exercise of counting postal ballots manually will itself take a couple of hours at least, an EC official said.

Out of the 543 Lok Sabha seats, elections were held in 542 constituencies as the EC had cancelled polls to the Vellore constituency on the ground of excessive use of money power.

The Union Home Ministry on Wednesday alerted all state chief secretaries and directors general of police about possibility of eruption of violence in different parts of the country during counting of votes.

"This is in the wake of calls given and statements made in various quarters for inciting violence and causing disruption on the day of counting of votes," a Home Ministry statement said.

It also asked states and UTs to take adequate measures for the security of strong rooms and venues of counting of votes.