Speaking at the opening ceremony, Chinese President Xi Jinping emphasized OBOR's international credentials in the face of criticism that the project will be dominated by Beijing.
"What we hope to create is a big family of harmonious co-existence," Xi said, adding that all countries were welcome to take part in the project.
Xi announced an additional $124 billion in funding for the OBOR initiative, including loans, grants and $8.7 billion in assistance to developing countries. According to Chinese state media, some $1 trillion has already been invested in OBOR, with another several trillion due to be invested over the next decade.
Addressing the forum after Xi, Russian President Vladimir Putin appeared to take aim at the US, which is not involved in the OBOR initiative.
"Protectionism is becoming the new normal," Putin warned, adding that the "ideas of openness and free trade are increasingly often being rejected (even) by those who until very recently expounded them."
OBOR, which has been in the works for four years, spans more than 68 countries and up to 40% of global GDP. It is China's push to put it in a position of world leadership as the US under President Donald Trump takes a more protectionist approach and gives up the mantle of globalization.
Sunday's forum is being held near Beijing's Olympic Park -- the site of the 2008 games -- as the city enjoys the type of splendid weather China's leaders have shown themselves adept at creating on demand
when needed for political events. Roads around the venue have been closed down amid a heavy security operation.
In attendance Sunday were Chinese President Xi Jinping -- whose personal project the OBOR initiative is -- Russian President Vladimir Putin, Turkish President Recep Tayyip Erdogan and Philippines President Rodrigo Duterte, alongside a host of other world leaders and top ranking officials.
Early Sunday, North Korea launched a ballistic missile
, emphasizing how high tensions in the region are at the moment and stealing focus from the OBOR forum in what could be seen as a deliberate insult to Xi.
The leaders of the US and most European economies were notably absent Sunday. While the US sent Matt Pottinger, special assistant to the President, no cabinet or elected officials were in attendance.
In a communique announcing a new trade deal with China
Thursday, the US said it "recognizes the importance of China's One Belt and One Road initiative," but Washington is largely uninvolved in OBOR or connected projects like the China-led Asian Infrastructure Investment Bank (AIIB).
Speaking to CNN Saturday, AIIB President Jin Liqun was positive that the US could still play a role in China's projects, saying that "regardless of the membership of the US ... we can work together."
"The door is open, any member is welcome to join," he added.
While OBOR has been hailed within China as something that can benefit the whole world and lift millions out of poverty, further afield its reception has been more mixed.
Jörg Wuttke, outgoing president of the EU Chamber of Commerce in China, warned last week that
the initiative has increasingly "been hijacked by Chinese companies, which have used it as an excuse to evade capital controls, smuggling money out of the country by disguising it as international investments and partnerships."
He and other critics have pointed to restrictions on and obstacles to foreign firms doing business in China as evident of the hypocrisy behind Beijing's grand unifying vision.
Even neighboring India has been skeptical. The country's finance and defense minister Arun Jaitley told reporters
this month Delhi has "serious reservations" about the project, particularly regarding China-funded development in Pakistan-administered Kashmir.
US pulls back
While many countries may have gone into OBOR with a "more rosy tinted view of what China's intent was," the scales are increasingly falling from their eyes, said Christopher Balding, a professor of economics at Peking University.
Of particular concern for many is what happens if Chinese-funded projects fail. In the past, this has meant Chinese firms or banks "essentially taking over," Balding said, giving them complete control over very strategic projects in foreign countries. Some have also warned of projects becoming expensive white elephants with little payoff for backers or locals.
Jin said such warnings are "necessary," adding that in the past "there were white elephants, there were mistakes."
"It's very important that the resources put into (OBOR) projects must be producing tangible results for the people" of the countries they are in, he told CNN.
Max Baucus, a former US ambassador to China, said OBOR has "if not frightened, then at least concerned, a lot of countries along the way."
Prior to Donald Trump's election as US President, it could be expected that Washington's Trans-Pacific Partnership (TPP) -- a free-trade alliance of 12 Asian and Pacific economies -- would act as something of a counterbalance to rising Chinese power.
Trump however, pulled the US out of the deal a day after taking office
. While it still includes Australia and Japan, both major economies, without Washington's backing the TPP will be far smaller if it manages to nevertheless go ahead.
The US has also reduced activity in the hotly contested South China Sea
, in what has been seen as another concession to China by the new US president who hopes for a solution in North Korea.
Baucus said the country's withdrawal from the region risked creating "a vacuum."
"(TPP was) an economic complement to military planning in the South China Sea," he said, while OBOR puts China "in the driving seat."