Taiwan is one of China's most sensitive issues and a potentially dangerous military flashpoint.
China's hostility towards Taiwan has risen since the 2016 election of President Tsai Ing-wen from the island's pro-independence Democratic Progressive Party.
China suspects Tsai wants to push for formal independence, which would cross a red line for Communist Party leaders in Beijing, though Tsai has said she wants to maintain the status quo and is committed to ensuring peace.
China has been infuriated by U.S. President Donald Trump's signing into law legislation last week that encourages the United States to send senior officials to Taiwan to meet Taiwan counterparts, and vice versa.
The United States does not have formal ties with Taiwan but is required by law to help it with self-defence and is the island's primary source of weapons.
In a speech at the end of China's annual session of parliament, Xi told the 3,000-odd delegates that China would push for the "peaceful reunification of the motherland" and work for more Taiwanese to enjoy the opportunities of China's development.
"It is a shared aspiration of all Chinese people and in their basic interests to safeguard China's sovereignty and territorial integrity and realise China's complete reunification," Xi said.
"Any actions and tricks to split China are doomed to failure and will meet with the people's condemnation and the punishment of history," he added, to loud applause.
China had the will, confidence and ability to defeat any separatist activity, Xi said.
"The Chinese people share a common belief that it is never allowed and it is absolutely impossible to separate any inch of our great country's territory from China."
China has also been worried about independence activists in the former British colony of Hong Kong following big street protests there in 2014 calling for universal suffrage.
Xi said China would uphold Hong Kong's high degree of autonomy, but would also seek to increase "national consciousness and patriotic spirit" in the financial centre.
Taiwan has shown no interest in being run by China, and has accused China of not understanding how democracy works, pointing out that Taiwan's people have the right to decide its future.
The new U.S. law on Taiwan adds to strains between China and the United States over trade, as Trump has enacted tariffs and called for China to reduce its huge trade imbalance with the United States, even while Washington has sought Beijing's help to resolve tension with North Korea.
Taiwan has thanked the United States for the law and its support, but its foreign ministry said on Monday there were no plans for any senior leaders, such as the president, to visit the United States.
While stepped-up Chinese military exercises around Taiwan over the past year have rattled the island, Xi reiterated China's assertion that its rise was not a threat to any country, though China considers Taiwan to be merely a Chinese province not a nation.
"Only those who are in the habit of threatening others will see everyone else as a threat," Xi said.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)