Tsai, who posted the message to her Facebook account on Sunday evening, said the duty of democracy is for all.
"In every era there is a mission for oneself," Tsai wrote, according Taiwan's Central News Agency. "[My] current mission is to defend the democracy of Taiwan."
The Taiwanese leader added democracy should never be taken for granted in a statement that appeared to compare Taiwan's political system to authoritarianism on the mainland.
"Democracy is not something that falls from the sky but the result of efforts of Taiwanese citizens," Tsai said. "We secured the freedom of press and a democratic system because of our democratic forbearers who brought threats to it knees under martial law."
Tsai called on all Taiwanese to speak their opinions freely on the Internet and encourage each other through social media "likes."
The president has responded previously to Xi, who had proposed last week Taiwan should accept "peaceful reunification" with the mainland as inevitable. Beijing reserves the right to use force to achieve such an aim, Xi had added.
Tsai has rejected talks with Xi on unification, but said Saturday she was willing to talk if China renounced the use of force against Taiwan, the South China Morning Post reported.
"By emphasizing 'one China' and 'one country, two systems,' particularly in the context of the so-called 1992 consensus, China has made clear their political intentions towards Taiwan and their steps for unification," Tsai said. "This is a major disregard for the fact that the Republic of China, Taiwan does exist, and is in full operation like all other democratic countries."
The 1992 consensus refers to an understanding there is only one China, but with flexibility in interpretation of what "China" means to different parties.