Cross-strait issues on the part of Taiwan should be handled by the elected government which is monitored by Taiwanese people, said President Tsai.
At a gathering commemorating the 40th anniversary of issuing the Message to Compatriots in Taiwan that marked a thaw in China-Taiwan relations, Xi said, emphasizing that "Chinese do not fight Chinese" that peaceful reunification served the interests of his compatriots.
"I want to appeal to China that it must admit to the reality of Taiwan's existence and must respect our 23 million people's insistence on freedom and democracy".
"We are willing to create broad space for peaceful reunification, but will leave no room for any form of separatist activities", Xi added.
In his speech on Wednesday, Mr Xi sent a warning to advocates of Taiwan's independence, who include supporters of Taiwan President Tsai Ing-wen.
"Deviating from the one China principle will result in tension and turbulence in cross-strait relations, harming the interests of the Taiwanese compatriots", Xi said.
'It's rather empty and doesn't have any new points except that cross-strait unification would not affect the interests of other countries, ' said Fan Shih-ping, political analyst at National Taiwan Normal University, adding that Xi's words may also be intended for the U.S., which views Taiwan as part of its Indo-Pacific strategy. Her remarks signaled that she would continue to take a firm line toward Beijing despite her recent election losses to Taiwan's more Beijing-friendly Kuomintang, Chiang's former party.
"We make no promise to renounce the use of force and reserve the option of taking all necessary means".
Mr Xi, meanwhile, is under pressure from growing criticism within the party over his handling of foreign affairs - notably strained relations with Washington, whose moves to support Taiwan have drawn China's ire.
"No one and no force can change the fact that Taiwan is part of China, and the historical and legal fact that both sides of the strait belong to one China".
Taiwan's leader rejected his call just hours later.
At the same time, Taiwan has developed a vibrant democratic system and a deepening sense of local identity that is distinct from China - and, for some, hostile to the authoritarian rule of the Chinese Communist Party.
China, meanwhile, has sought to isolate Taiwan under Ms. Tsai, freezing official cross-straits communication, increasing military exercises around the region and persuading a series of diplomatic allies to cut ties with Taipei.
To accommodate differences in Taiwan's political system and civil society, China has proposed adopting the "one country, two systems" policy, which was implemented in Hong Kong after the British handed the city back to China in 1997.
On Jan 1, 1979, China stopped decades of regular artillery bombardment of Taiwan-controlled islands off the mainland.
However, the offer was rebuffed by Taiwan's then-president Chiang Ching-kuo, who in April that year came out with a "three noes" policy of no contact, no compromise and no negotiation with China.