The worst corruption crisis in football history stems from the governing body's decision to award Russia and Qatar the next two World Cup tournaments, Fifa president Sepp Blatter has said.

Mr Blatter spoke at the Fifa congress (follow live broadcast here) hours before the presidential election in which he is expected to be reelected to a fifth term. He has refused calls to resign after Fifa was targeted by US and Swiss authorities in separate corruption investigations.

In 2010, Russia was chosen to host the 2018 World Cup and Qatar was awarded the 2022 tournament amid widespread allegations of wrongdoing.

"If two other countries had emerged from the envelope, I think we would not have these problems today," Mr Blatter said. "But we can't go back in time. We are not prophets. We can't say what would have happened."

The United States was one of the losing bidders for the 2022 World Cup. On Wednesday, US authorities indicted 14 people on bribery, racketeering, fraud and money-laundering charges going back to the 1990s.

Seven of the officials - including two serving Fifa vice presidents - were arrested in Zurich on Wednesday ahead of the congress.

"I am not going to use the word coincidence but I do have a small question mark," Mr Blatter said about the timing of the raid.

When the congress broke for a lunch a few hours later, a bomb threat was made at the venue hosting the meetings, according to Swiss police.

"An anonymous threat against the Fifa congress was received," Fifa secretary-general Jerome Valcke said when the proceedings resumed.

The two criminal investigations have cast a new shadow on Mr Blatter's 17-year reign as president of Fifa. The 209 Fifa member nations will decide later today whether to stick with the 79-year-old Mr Blatter or vote for Prince Ali bin al-Hussein of Jordan.

Amid the dramatic build up to the election, Uefa president Michel Platini appealed to Mr Blatter to immediately step down - calls the president has rejected.

"I am willing to accept the president of Fifa is responsible for everything but I would at least like to share that responsibility with everyone," Mr Blatter said in a presidential address this morning. "We cannot constantly supervise everyone in football ... you cannot ask everyone to behave ethically."

Mr Blatter also cautioned that "it will take some time" to rebuild Fifa's reputation.

"The events of Wednesday have unleashed a storm and there was even questioned whether this congress would be organised or change the agenda," Mr Blatter said.

"Today I am appealing to unity and team spirit so we can move forward together. That may not always be easy but it is for this reason that we are here together today."

A two-thirds majority would be enough for Mr Blatter or Prince Ali to win the secret presidential vote, or a simple majority in a second round of voting.

Just forcing the ballot to a second round could represent a victory of sorts for Mr Blatter's critics, denying the incumbent president an emphatic mandate in his next term.

Mr Blatter's opening address to the congress was briefly disrupted by a pro-Palestinian activist, who held up a red card and shouted "Red card to racism". The protest was to draw attention to a campaign that aims to stop Palestinian players from being detained by Israeli security forces.

After calling for security, Mr Blatter announced: "I would ask you to please check the access points of this room."

Meanwhile, UK Prime Minister David Cameron said Mr Blatter must resign and "the sooner that happens the better".

Asked about the Fifa scandals at a joint press conference in Berlin with German chancellor Angela Merkel, Mr Cameron said it was "unthinkable" that Mr Blatter was the right person to take the world football body forward after this week's corruption accusations.

Mr Cameron added: "Frankly, what we have seen is the ugly side of the beautiful game."

Ms Merkel called for Fifa to make a clean break with corruption but stopped short of saying that Mr Blatter should resign.

Also today, Swiss police said a bomb threat was made against the venue where the congress is being held. Zurich city police spokesman Peter Sahli said a police operation was ongoing.

An AP reporter at the scene said the Hallenstadion's concert hall auditorium was cleared but the building itself was not evacuated.

Fifa secretary-general Jerome Valcke said "an anonymous threat against the FIFA congress was received". The meeting then resumed after lunch.