The Prime Minister was in New York for five days during which he addressed the UN General Assembly`s annual session and held talks with several world leaders.
Singh, who turned 79 yesterday, celebrated his birthday on board Air India One on his way to Frankfurt from New York.
In his address to the General Assembly on Saturday, Singh made a powerful call for early reform and expansion of the UN Security Council "to reflect contemporary reality" and asked nations not to allow the global economic slowdown to trigger protectionism through various barriers.
On terrorism, he had said there cannot be "selective approaches" in dealing with the scourge that needed to be fought across all fronts.
He had also expressed India`s "steadfast" support for Palestinians` struggle for a sovereign, independent and united state, saying that New Delhi looked forward to welcoming Palestine as an "equal" member of the United Nations.
On the sidelines of the UN General Assembly session, the Prime Minister held talks with Iranian President Mahmoud Ahmadinejad and Sri Lankan leader Mahinda Rajapaksa.
He had also met the newly elected Prime Ministers of Japan and Nepal -- Yoshihiko Noda and Baburam Bhattarai respectively.
The Prime Minister had attended the UNGA after a gap of two years.