BEIJING—China's aggressive response to plunging share prices undercuts its pledge to have the market play a decisive role in the economy and risks cementing investors' belief that Beijing will always bail them out.

Over a tense weekend, the government oversaw several steps to stem the selling frenzy that has seen the benchmark Shanghai Composite plunge 29% in three weeks. Brokerage firms, mutual-fund managers and a government investment arm pledged to buy stocks, new share offerings were suspended, foreign stock quotas increased, and the central bank vowed to provide liquidity to help investors borrow to buy shares.

While the flurry of rescue moves may stem the short-term panic, some economists warned it could encourage another asset bubble if not implemented carefully, as investors are likely to conclude that Beijing will make good on even reckless investments.

"You could've been excused as a punter for thinking there was no downside risk here," said ING Group economist Tim Condon.

The Shanghai Composite Index opened 7.8% higher Monday and it looked like the all-out government mobilization would decisively lift stocks. But the index quickly pared those gains. It ended only 2.4% higher and even spent part of the day in negative territory. Investors and analysts said heavy buying of major blue chips like state-owned banks and energy firms late in the session appeared to be the work of state-backed funds. The smaller Shenzhen Composite fell 2.7%.

China is hardly the first to move in when markets tank. But some economists say leaders may have overreacted as systemic risk in China from the recent stock plunge is low, and far from comparable to the volatile events that had set off the 2008 global financial crisis and prompted the U.S. Federal Reserve to pull out all the stops.

"They probably panicked," said Socié té Gé né rale economist Klaus Baader. He questioned the wisdom of directing monetary policy at stock markets. "A market in which authorities intervene is very dangerous. If the market wants to correct and you throw spanners in the works, it breeds instability."

Shanghai shares are still 50% higher than at the start of the rally and up 14% this year. Stock-market investors are a small segment of Chinese society. Banks, at least on paper, are not hugely exposed. And many of those holding large margin positions are high-net-worth individuals, economists said, making a collapse in consumer spending unlikely.

Well-functioning equity markets are fostered with policies that strengthen the financial and economic architecture, not with those that meddle with prices, some economists said.

"China needs to first strengthen its supervision over margin financing, especially those outside of regulatory control," said OCBC Bank economist Dongming Xie. "Then it needs to boost economic growth, the foundation of a healthy stock market."

China's dramatic bid to put a floor under the selloff may have been shaped by a desire, after encouraging the market to appreciate, to appear in firm control of the economy, some said.

"Many say it's a systemic risk," said Industrial Securities Ltd. chief strategist Zhang Yidong. "I believe it's more political risk. The top authorities are fretting that the sharp pullback could lead to social unrest."

The rout has also threatened China's bid for the sort of well-capitalized market that can allow struggling state-owned companies to replace debt with equity.

Much of the state funds being funneled into the market could end up expanding government ownership since much of the buying is likely to go toward buying shares of state firms given their relatively low price-equity ratios and reputation as stable investments. And the move by China's central bank to indirectly help investors borrow to buy shares could worsen the risk from margin financing that authorities had tried to stem, economists say.

Henry Luo, a 24-year-old student and new investor from Hunan, said he was heartened by the government's supportive moves Monday. "Markets kept falling because many people lost confidence," he said. "I'll sell if I can recoup my money. Otherwise I'll leave it in the market as a long-term investment."

Among the options open to Beijing in coming weeks if markets continue falling include directing state-owned companies to buy more of their own stock and deploy more central bank funds for share buying, economists said. Also possible: another broad interest-rate cut and reductions in required bank reserves after last month's double cut.

As part of confidence-boosting measures, asset-management units of leading security firms have pledged to hold shares until the Shanghai index reaches 4500, some 700 points over Monday's 3775.90 close. Some read this as a de facto government target, raising concerns. "At that level, valuations will be relatively stretched again," said Cré dit Agricole CIB economist Dariusz Kowalczyk. "That's the trouble with setting a target. Investors will see it as a good level to sell into."

Beijing's effort to make its markets more attractive to international investors and position the yuan as an international currency may also have been set back this weekend, some said.

"They need to have a market that's reasonably transparent, that works by the rules most markets work by, not facing intervention by policy markets," said Socié té Gé né rale's Mr. Baader. "This destroys confidence in the market and does long-lasting damage in attracting international capital. That's not going to endear the market to global investors."

Grace Zhu and Rose Yu contributed to this article.

Write to Mark Magnier at mark.magnier@wsj.com

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