Explosions at the Boston marathon finish line on Monday killed three people and injured 176. No arrests have yet been made.
The London race, first run in 1981, will attract 36,000 runners on Sunday, among them Britain's double Olympic champion Mo Farah, who will be cheered on by hundreds of thousands of supporters lining the course.
In a weekly radio phone-in for LBC, Clegg said the images from Boston had been heartbreaking.
He said he had personally contacted the head of London's Metropolitan Police about the safety of the race, which winds round landmarks like Tower Bridge and goes through the Canary Wharf financial district before ending in front of Buckingham Palace.
"I have spoken to the Met Commissioner himself about this yesterday, and I have a huge amount of confidence in the police and security services of this country," Clegg said.
"They do an amazing job keeping us safe all the time. They are doing a double, triple, quadruple checking of all their arrangements for the marathon, because of course what happened in Boston is just horrific."
Home Secretary Theresa May confirmed that security would be stepped up along the 26-mile route.
"They have made some adjustments to the policing arrangements for the marathon and they have put some increased policing in," she told a committee of legislators.
The race will be Britain's second big security test in the space of a few days after Wednesday's ceremonial funeral of controversial former Prime Minister Margaret Thatcher passed without incident.
- Politics & Government